If she's not interfering with your life in her religious practices, I see no problem. She is a teen after all, so she has to check things out.
Now, if she has to eat stuff I won't/don't/never gonna happen, since she is state-placed, I'd think the state ought to pay for her choices. Say, she has to eat dandelions and a fatted lamb. Sorry, no can do. Dandelions taste disgusting, and I can't afford a lamb, once more do I want to have to cook the animal. That kind of thing. Not, something small like chitlins. (Don't ask, if you don't know. lol) But one of the kids in our group home wanted chitlins, so we had chitlins. He was eating it alone the next time though. lol
If she has to get out a rug and pray, two places she's not doing that -- in the middle of my kitchen and in the middle of my living room. Mostly because it interferes with foot traffic, and if that's where a person wants to do that, they're doing it for show and nothing else. But in her bedroom? Not a problem. I wouldn't even mind in the hallway as long as she's not blocking off the bathroom. (I'm very practical. lol)
And then in the right time, I would talk to her about her choices. More of a hear her out, than a "my way or the highway" talk. But, there would definitely be a cross-exchange on my views vs. her views. Ultimately, her decision, but the only way to make a good decision is to understand the decision being made.
I'd never force anyone into Christianity. (Well, not exactly true. I would, if I could, it's just that I know forcing someone is going to stop them from wanting Christ.) But rest assure, what I believe about God would come out, and does come out any day of the week. Whether someone hears it or not, I can guarantee God is working it out his own way, even if I never see the end result.
Isn't that about what you signed up for when choosing to become a foster mom. How likely did you expect the chances of seeing end result were, since she's a teen and will be living on her own at 18?