The use of Charm Person in place of or to aid Diplomacy

[LFDPrivateer]

The rules of the Charm Person are straight and to the point:

Abbreviated Spell info from Core Rulebook:

School enchantment (charm) [mind-affecting]
Casting Time 1 standard action
Components V, S
Target one humanoid creature
Duration 1 hour/level
Saving Throw Will negates; Spell Resistance yes

This charm makes a humanoid creature regard you as its trusted
friend and ally (treat the target’s attitude as friendly). If the
creature is currently being threatened or attacked by you or your
allies, however, it receives a +5 bonus on its saving throw.
The spell does not enable you to control the charmed person as if
it were an automaton, but it perceives your words and actions in the
most favorable way. You can try to give the subject orders, but you
must win an opposed Charisma check to convince it to do anything
it wouldn’t ordinarily do. (Retries are not allowed.) An affected
creature never obeys suicidal or obviously harmful orders, but it
might be convinced that something very dangerous is worth doing.
Any act by you or your apparent allies that threatens the charmed
person breaks the spell. You must speak the person’s language to
communicate your commands, or else be good at pantomiming.

My question is, what happens at the end of the spell? Is the NPC or PC aware that a spell took place?
and finally
Can a PC or NPC uses Charm Person in place of/or to aid a Diplomacy Check to improve the attitude of a PC or NPS?

Relating to an Adventure Path:

I have a witch in the AP Skulls and Shackles who would like know know if Charm Person can be used to help gain the favor of fellow crew members. Honestly I don't know how to answer this. I'm leaning toward a bonus to the roll depending on the original attitude but would like some advice or insight.

[Wriggle Wyrm]

I would use some sort of internal sense motive check, with modifiers based on how they felt about the guy beforehand.

Any way you decide to do it though, you should warn the player beforehand that by using mind control on his own allies, he's playing a very dangerous game. After one successful sense motive check or will saving throw, he should pretty much be marked for death by the offending party and possibly the rest of the crew. Also considering the rather dishonest nature of what he’s doing he shouldn’t expect to be called out on it so much as lynched, stabbed in the back or murdered in his sleep.

[Morgen]

What happens at the end of the spell has quite a lot to do with what happened during it. If the spell caster was using it as an ice breaker and things went agreeably, the charm victim might retain any attitude adjustments made due to diplomacy, or simply still be on good terms with them. Treated poorly however would likely cause proper reactions and confusion, even (renewed) hostility towards the caster.

If the world your in has a very common amount of knowledge about magic, people might be able to puzzle out what happened, otherwise there would need to be some actually knowledge of magic to understand a charm spell's effects. So you'd have to have some knowledge arcana or spellcraft to really understand what happened.

A good baseline would be to look at any legal repercussions that may exist in the world for the use of that type of magic. If there are laws against exploiting people by using charm spells, then it would be more likely for people to know what those spells are and do then in a world that lacks that kind of legislation.