I'm pretty sure you can't multiclass out of the monster class until you finish it, although I think he has a feat that lets you.
It is in fact explicitly allowed. you are not ussualy allowed to combine monster classes.
The regeneration ability should have also said you lose it if you become immune to nonlethal damage. Even if it did though, a single level of the pugilist fighter variant (Dragon 310) lets you subtract your con modifier from all nonlethal damage you take. Also, apparently a 75% chance to negate critical hits doesn't conflict with your regeneration.
If my player did that I would say that there is a 75% chance of the damage bypassing regeneration
Also consider a level in crusader since you'll be in mellee anyway some maneuvers should make it a lot more entertaining. I'd take crusader as the 9'th level so you start with 3'rd level maneuvers.
I have a tarrasque player in my party, look at the grab feats in serpent kindoms to reduce the grapple penalty for not being considered grappled form -20 to 0. I will not though that I banned the second feat for now, though I might allow it later.