Sounds reasonable - he's a werewolf and wants it as a kind of regeneration thing. So I gues, he has to stop and breathe for a moment, then the self heal kicks in.

The Monstrous Move UV saiys:
Unquenchable Vitality: When you have taken harm, you can heal yourself. Roll +Cool.
On a 10+, heal 2-harm and stabilise your injuries.
On a 7-9, heal 1-harm and stabilise your injuries.
On a miss, your injuries worsen.

I'm not a native speaker, but I read it like every time I as a Keeper do a harm move on the monstrous, directly after that (as a reaction), he may roll for UV, right? My player said, it would be too powerfull (actually very rare a player says that^^). But I guess it's more or less like one point of armor - sometimes better, sometimes worse...