[Always *]

Yeah I agree with the people who responded above me, there are other ways to hide the fact she's not straight than using a guy to do so. Simply not telling people about that part of her life for example and simply not having a public boy/girl-friend is a start.
It really isn't fair for you to have to be the one getting hurt and having that sort of stress caused to you just cause she wants a cover. She can SAY she likes boys, I know A LOT of straight girls even who are my age (university aged) and they've never had boyfriends and no one thinks anything of it, so it's really not necessary for her to take it that far. I know she might feel like she has to work harder since she has to hide it but she doesn't.
Think about the pressure she must be under though. Her mom is basically telling her that she doesn't love your girlfriend enough to accept her for who she is and that she WILL NOT let her be a part of her own mothers life if she comes out. Can you imagine how devastating that is for her? Sure, it's cause she thinks its a sin, but can you imagine if you thought some quote un quote "sin" was more far important than unconditionally loving your child? Cause I sure as hell can't. One of my friends was kicked out of HER house for choosing to live a different life than what her family did, and no it was nothing bad, her mom was just VERY religious, and they have a difference of opinion in how to live and she got kicked out for it. So I have seen what it can do to a girl being kicked out like that. I know that her and her mom are very close now that her mom is "over it" but it still doesn't change what she went through then. So I DO think that some understanding about how your friend must be feeling about her moms view there should be taken into consideration.
But that being said, I do think it is more important to talk to her about it, let her know that trying to hide the relationship is hard enough but now her having cover (and apparently actually kissing him and stuff by the sounds of it) is killing you. She needs to understand that it can't (and shouldn't have to be) like that just so her mom won't know. She could always just like "make up" a boy or talk about boys or SOMETHING and bide her time until she can actually come out to her mom when it WON'T get her kicked out. A lot of people are gay (lesbian bi etc) and only come out AFTER they leave home to spare themselves and their family all that drama of the dinner table fights (ok, it's not only at the dinner table but thats not my point). I know a lot of families accept it though. And maybe you guys can tell more people at school or something, make it a bit more easy. Unless of course, that might mean mommie hears. But she needs to know you love her (really care about her?) and that you will support her. And maybe you should talk to your mom about letting her stay there for a while should her mom find out, that way you can at least reassure her that like yeah you've got somewhere to go kinda thing IF worst comes to worst for her