Is my ex-girlfriend's boyfriend mentally crushing her?

Maybe he isn't pushing his religion on her, he's just sharing his "faith", because that's what Christianity is. Not just another religion. Maybe she's found God, and isn't crazy like you think. Just because your an atheist, doesn't mean she has to be. And seeing as she's your "ex" girlfriend, you should just leave her alone, because she's not yours anymore. I'm sorry. And poisonous? There's nothing poisonous about being a Christian; if anything it's liberating. Just let her be happy, maybe even follow her example. Live you life, dude.