Two easy solutions for your problem dude.
1. Tell your girlfriend she's an idiot, and even if In Flames had anything to do with satan, they'd still rock her face off anyway.
2. Immediately dump the stupid woman, nless she gives good head of course. Seriously, if she wants to assume they're a satanic band, then she needs to give the lyrics another look. The lyrics are about our society and mostly about our society and the problems with life. Nothing satanic.
Tell her to listen to Dimmu Borgir, then maybe she can go on about Satanic... Even better... Vomitory. =D