Easy answer: He stops being an occult expert.
Many people (and characters) go through several jobs in their lifetime. If he is content with his character at large, but doesn't enjoy the occustist angle, have him swear off studying the occult and pursuing a new line of work. I realize Mutants and Masterminds works a lot differently than your standard D&D, so "multi-classing" is much more difficult. How about this:
The character, after having been exposed to Lovecraftian craziness, petitions the "powers that be" to erase the memory of all that he has learned. Mechanically, this allows his to buy back the points he spent on occultism and beings his life anew with a new trade. Happens in TV all the time... why not RPGs?
Hard answer: I'm REALLY out on a limb here, but maybe the problem isn't with the gf, it's the occultism. Perhaps EVEN the gf is hardcore religious and now, finding out that her bf pretends to be a demonologist et al, has laid the hammer down on his character concept. "Either you stop playing that EVIL game, or we're through!" Perhaps he's having a personal crisis of faith. Bottom line -- it's a game, and nothing is written in stone.