Taking these in reverse order:
Detect Evil reads, in part:
You can sense the presence of evil. The amount of information revealed depends on how long you study a particular area or subject.1st Round: Presence or absence of evil.
2nd Round: Number of evil auras (creatures, objects, or spells) in the area and the power of the most potent evil aura present.
If you are of good alignment, and the strongest evil aura's power is overwhelming (see below), and the HD or level of the aura's source is at least twice your character level, you are stunned for 1 round and the spell ends.
3rd Round: The power and location of each aura. If an aura is outside your line of sight, then you discern its direction but not its exact location.
Assuming other "Detect" spells work similarly, that tells me the caster would detect the presence or absence of the alignment of the PC on the first round, would know the count including the PC on the second round, but on the third round would be unable to locate the PCs aura but would be able to determine the direction because they are presumably inside the Mimic and therefore not in direct line of sight. If the Mimic is disguised as a large chest though, this would immediately rouse suspicion as they should rightly presume there is someone of the appropriate alignment inside the chest.
If the Mimic is disguised as a humanoid, since they are Neutral, detecting them as non-Neutral would probably just lead to the caster believing the disguised Mimic is a humanoid of that alignment.
True Seeing reads as follows in the second paragraph:
True seeing, however, does not penetrate solid objects. It in no way confers X-ray vision or its equivalent. It does not negate concealment, including that caused by fog and the like. True seeing does not help the viewer see through mundane disguises, spot creatures who are simply hiding, or notice secret doors hidden by mundane means. In addition, the spell effects cannot be further enhanced with known magic, so one cannot use true seeing through a crystal ball or in conjunction with clairaudience/clairvoyance.
So True Seeing would see that the Mimic is not really a chest or a box or even the humanoid they are disguised as. But it would not see the PC hidden inside.
Regarding healing, it would depend on the spell or effect. Cure Light Wounds, for example, has a range of Touch. If the PC is entirely within the Mimic, the healer would have no way of touching the PC, only the Mimic, unless the Mimic somehow knew to expose part of the PC. The ability of the Mimic to know to do this would entirely depend on what sort of communication and intelligence abilities you have granted it. If the CLW was from a wand, you would need to decide whether you are ruling that the wand needs to also touch the target or whether, by virtue of being a wand, it need only be pointed at the target. Even then, if line of sight is implied, like with the True Seeing example above the wand user would lack line of sight. Rule accordingly.
Finally, with regard to damage, you could rule this a number of ways. According to the SRD:
Mimic Object (Ex) A mimic can assume the general shape of any Medium object, such as a massive chest, a stout bed, or a door. The creature cannot substantially alter its size, though. A mimic's body is hard and has a rough texture, no matter what appearance it might present. A mimic gains a +20 racial bonus on Disguise checks when imitating an object in this manner. Disguise is always a class skill for a mimic.
This suggests that a Mimic may be "hard" enough to absorb incoming damage without transferring it to anything or anyone in the Mimic's interior, unlike armor. In fact, perhaps you could check out the rules for a Synthesist Summoner as the mechanics would, I would think, be quite similar.
All of that said... how does the PC breathe? Or see? Or move? A Mimic, according to the SRD, typically weighs 900 lbs. The PC is unlikely to be the one actually moving the Mimic. If the PC is entirely closed within the Mimic, they cannot see, they cannot breathe and unless they have sufficient room to move their mouth and air to breathe, they cannot speak and therefore cannot communicate normally with the Mimic. If a telepathic link is established, you could make some of it work (communication since a Mimic speaks Common, movement because the PC could essentially "think" movements to the Mimic) but I would suggest that such movements would be very ungainly, clearly not normal, greatly delayed. I would propose massive REF save penalties, no DEX AC bonuses, Disguise penalties offsetting some of the Mimic's Disguise bonuses due to the abnormalcy of movement (if disguised as a humanoid).
Furthermore, what sort of humanoid are they disguising themselves as? The SRD seems to allude to the Mimic being able to disguise itself as any man-made object of Medium size. While "humanoid" is not explicitly ruled out, I wouldn't have considered it likely. Still, if you as GM have ruled that it is possible, the Mimic entry *does* indicate that it cannot substantially alter its size. This means that any humanoid disguise should have roughly 900 lbs (plus the PC's weight) in weight. And again, even if you take into account an ability to compress some of itself in order to take on a smaller size, it still has to mask the PC's mass. Is the PC naked inside? Has a backpack? A sword? That's going to need to be part of the disguise or else the guard at the gate is going to have doubts about the wizard walking up who looks like he can bench press a team of horses and has an odd sword shaped bulge in his robe.
Is the Mimic going to carry the equipment instead? Who is going to wield the sword? Again, only with some sort of telepathy would the person inside be able to actually do any sort of control for the Mimic handling the weapon and then there would be massive penalties involved. And again, how does the PC see or hear, etc.
If I had to adjudicate this situation for one of my players, I would expect that hiding inside the Mimic could happen if the Mimic was formed into something inanimate as that is what I would rule that a Mimic is capable of. This would allow for static hiding to wait for danger to walk past without noticing the PC (again, assuming a detection spell is not used). But time "inside" would need to be factored in in terms of air to breathe and so on. And I would rule that a Mimic cannot become a humanoid figure of any realism. And even if they could, the size discrepancies would rule out most uses like infiltration and stealth activities.
Just my $0.02.
I'd have the mimic's natural acidity start destroying the pc's stuff.
I'd have the mimic's natural acidity start destroying the pc's stuff.
If you mean the adhesive ability, it does say that the Mimic can dissolve the adhesive at will. I would assume that Casper the Friendly Mimic would avoid gluing his buddy to Casper's guts as much as possible. Likewise he would avoid digesting him (if that is what you referring to). Still, good point. Heck, make the Mimic roll a Will save each round of combat or start secreting digestive/adhesive secretions just out of habit/excitability.
For the most part, I'm trying not to get into Wish-bending mode. You clearly have ruled that he can have this companion, so I am trying to avoid the worst possible twists you can apply to this. But I do think some things require dealing with and there are some pretty clear cut limitations involved that are harder to hand wave away.
Now while I have let this play out I see how this may have been a mistake
If you think you have made a mistake, explain this to the player. It sounds like this players previous campaign was a different DM? They can have their mimic back when they play that other campaign again.
If you're going to let the player keep it, then just insert a repeat of everything Quintessentially Me said here.
Dot.