[@Pathfinder] Well, ideally, with how I envisioned Nile's powers, the mutations stop, making him assume human form again, returning him to the state at which he would be if he didn't have the genes of Eve inside him.

If you'd like though, the mutations simply stop, at whichever mutation they were in when they entered the Null Aura, and he loses the extra physical power he gains from them. Not all but a part of it, you decide how much would be good. Or if you have something else in mind feel free to share.