Here's what I'd do:
I'd try to make Jane welcome. Really, I would. Frankly, I wouldn't concern myself with her reasons for joining the group. At least, not until a problem arose. "Judge not lest ye be judged" and all...
But... I'd also cordially and honestly explain to her the tenor of the group. Be up front that she might not like everything that's said at the table and the group won't change for her, however, she's still welcome. Leave the choice up to her. Be honest, but don't try to make the decision for her.
And... don't focus on her religion. This is a people issue. If she does play, and wants to play a cleric of Jehovah, make it clear that the game is about fantasy, and no real-world religions are used, out of respect for them.
Also, try to avoid speculating about her motives. No good can come of that. Maybe she has the best of intentions, maybe not. But she's important to a friend of yours, so at least give her the benefit of the doubt.
You can't prevent other people from behaving like pricks, but you can prevent yourself.