Should it matter whether it will die or "just be unhappy"? Neither should be acceptable to you or your friend.
Regardless, to answer your question...
Violet Gobys are brackish water fish that commonly reach 15" in captivity (up to 2' in the wild). They are a peaceful, shy, and fairly hardy species. However, they not tolerate ammonia in the least.
No, your friend could not keep one in a 10g tank. It would be very unhappy. And then it would die. As you already know, your 29g is much too small for an adult, let alone a tiny 10g tank.
I would urge you to convince her not to keep one unless she is able to provide for it's specific requirements, including a large tank (55+ gallons), brackish water, and the proper lighting, decor and tankmates. There are many interesting species, even some oddballs, that would be a better choice for her. You might want to point her towards dwarf puffers. Although I've never owned one, they seem quite popular around here and could work in a 10g.
Sorry for the bad news. Just trying to help.