He probably won't forget the moves, the names on the other hand... He might get them confused with the names of similar looking moves in taekwando, or the other way around. As long as he keep practicing at least once a month (even if it's just for a few minutes) he'll probably still remember it even after those 5-7 years. However, he'll have to sharpen his skills in Muay Thai after that period to get back to the level he's already at, or not of course depending on what level he's at.

If he doesn't practice it there's a chance he may forget the names of form and the sequence in which it is performed.

So my cousin took Muay Thai for 2 years, he knew all of the moves . But he wanted to go in a second martial art, so he's quitting Muay Thai for Taekwondo, till he gets a black belt. Will he forget his moves from Muay Thai? Because he said it will take him 5-7 years to get a black belt in Taekwondo if he worked really hard.