Aside from the various "don't do it" remarks you are bound to get, I advise that the player really know what they are getting into before they attempt to play a Mystic Theurge. Unless they are allowed to use early entry shenanigans, or be the Guildmaster from a certain Kintargo Opera house, their spellcasting is going to be delayed. Even then, delayed significantly on one side. They need to be sure they are ok with that and be serious enough to make smart choices on Spell selection and alternate means to assist the party.
All that in mind, the half-elf has the best alternate class features (multitalented) and racial trait (bifurcated magic) that helps make up some of the caster level loss (not spell level). Make sure to pick hexes that do not depend on your witch level (or that are still effective without a high level). Protective Luck and Cackle immediately spring to mind. Perhaps think about VMC (variant multiclass) options as well. Trading off some feats to get some scaling class features might end up being worth it.
I guess having two full spell casters based on charisma is a nice deal, but they aren't getting much else out of it. At 7th level, right as they hit their prestige class, they'll have a BAB of 2, charm hex and one other hex, an animal companion, a familiar, some nature stuff, and the ability to trick animals into thinking they could have food or sex. And a bunch of first and second level spells.
Power wise, they're competing with 4th level spells and it doesn't look good. Their best bet is probably to lean on that animal companion as much as they can. Pick up the boon companion feat to keep it maxed out till they get to this level at least. It'll be a struggle.
Oh, and Deathless meant multidisciplined I think.
Do you know what their intentions with this character are?
Generally the trade off you get with MT is more spells per day and more spell variety in exchange for overall spell level and caster level. This sort of trade is usually considered a bad deal since typically as a caster you want your caster level to be as high as possible and you want access to higher level spells asap. The exact two things that are being traded away. Also, it's a half BAB class meaning it doesn't play well with martial builds that throw some magic into the mix.
Do they need access to all divine and arcane magic or is there just some mixture that they are looking for? If they just want to be caster type X with access to certain spells from caster type Y there are archetypes, prestige classes and feats that can do this without hurting your casting progression quite so much.
feyspeaker + seducer sounds like they're looking at casting lots of enchantment/charm type spells. Oddly enough, feyspeaker is one of those aforementioned archetypes as they can pilfer spells from the wizard list.
I'm trying not to just say "don't do it". So, any additional insight you have on the concept behind the build will make it easier to give advice.
I'll also point out that while traits like bifurcated magic are nice they don't tend to play well with other caster level boosting feats/items, since usually the goal is to raise your caster level above your character level and this is precisely when traits that boost caster level stop working.
Do they need access to all divine and arcane magic or is there just some mixture that they are looking for? (...).
This make me consult my spreadsheet. Witch has significantly more spells in common with the Cleric list than Druid does (528 vs. 300). Witch also has more spells on the Cleric, Druid, and/or Inquisitor list (769) than Druid has spells on the entire list (742).
So in addition to saying "don't do it because Mystic Theurge is a total trap option that severly weakens your character", I can also say "don't do it because Witch already does that job"!
Oh, and Deathless meant multidisciplined I think.
I did indeed. Thank you.
Adding a bit to my previous post, I have played a Mystic Theurge a few times in the past, with different class combinations. My absolute favorite is combining the Shaman and Sorcerer together.
The biggest thing I can stress for anyone who wants to play a Mystic Theurge is to understand that you will NOT be as good as a pure spellcaster in most areas, but that is OK. By taking up the mantle of Mystic Theurge, you have entered into a realm where you become somewhat of an ultimate support character. Whatever you can do to make that role stronger is something I encourage you to do. Most of the best option you have is with working with another character (or the whole party) to encourage them to make characters that compliment your own.
Bonded Mind + Share Spells will let you cast a number of those fun, personal spells you have on another player. Even at a somewhat reduced caster level (only -1 if you took bifurcated magic and multidisciplined as a half-elf), those spells will greatly enhance a full BAB class. Combine that with VMC Bard and now you have the ability to Inspire them further.
You can go as far as picking up an animal companion (via feats) and focusing on making that animal powerful enough to take your place in the melee or divert attention away from you. Have it support the others while you throw around buffs EVERY fight (because you have the spell slots to do so). If you worship Calistra (and dont happen to have a familiar), get that wasp familiar. Have it deliver touch spells for you.
As a Mystic Theurge, you are greater than the sum of all of your abilities. But that depends entirely on how clever you are and how prepared you are.
Mystic Theurge has an infamously bad valley of suck, but it's actually pretty solid at higher levels. I'm of the opinion that it's solid at 10th level and above and you don't really need to do anything special to make it work well. It's the lower levels as it's qualifying and immediately after qualification that it struggles.