Thanks Dodi! However, it still doesn't make sense b/c his skills are lower than hers! They both have charisma 3, she has fitness 8 and he has fitness 7. They both have various other skills. He is able to mentor while she is not. Maybe I just have a weird game, LOL
Edit: I took her to the gym one more time. She was able to mentor one sim, but when she left her punching bag and then came back my sim had to introduce herself to the other again. One other sim I clicked on had the option for my sim to "train w/him."