Hi,
I am in (roughly) the same situation as you are (atheist dating a JW).
Your girlfriend is a very special kind of JW if she believes in evolution and are at all interested in a non-jw let alone an atheist; this suggests she is not fully "in" her religion but has doubts and may leave (or fade out) of the religion in the future. I wouldn't just advice you to leave her (that's certainly not what I did!) but there are some things I would worry about if I were you: (I assume your girlfriend is baptized; that's a very important piece of information so you should ask her about that)
* Why is she not telling you the truth about her religion? She might be embarrassed or whatever, but it seems to me to be a pretty big thing to not mention. Are you sure she actually believe in evolution? (JWs accept something akin to "change within a kind" and that the earth is 4 billion years old; they just reject common descend and believe man has only been here for about 6000 years).
* If you two stay together it *will* hurt your girlfriend one way or another:
- If she stays in, you are in for some difficult times later on. She will in various ways be made aware she choose the wrong partner, and if your girlfriends family are hardcore you can expect them to put you on the shit list at least until you are married and possibly later. She will be expected to bring up her children "in the truth" (i.e. indoctrinated), not to celebrate birthdays. This will lead to conflicts later on.
- If she leaves (not disassociation but rather just stop goes to meetings), she will be viewed as a bad apple by her family. If her family are "good jws", you should not expect that her family wants anything to do with you until you are married, in fact you should not expect them to show up to your marriage. If she does anything "wrong" (like you two living together), she will be disfellowshipped and not see her family again. Most likely, even if you are married you will still have to hide things from her family because she will still be expected not to celebrate birthdays.
Basically, if you stay together, you guys are in for a giant multi-year shit show no matter what!
If you decide to stay together, I think there are a few things you should do:
1) Don't go Richard Dawkins on her, an in particular don't criticize her religion in such a way she can tell you have searched out ex-jw literature. If she says something you disagree with, try to limit your comments to "common sense" objections ("well, I couldn't imagine shunning my child no matter what"), be humorous and be non-assertive.
2) Work on your communication. Where does she stand in relationship to you? How can you make her comfortable talking about her religion? (ask her). Don't read to much into her living with her mum as she is in a very tough spot.
3) Be aware that if she is baptized (and probably also if not), if she does something "bad" you can expect everyone she knows to rat on her, including herself. In particular, *noone can know* you have had sex or lived together.
4) Educate yourself about cults. Standard atheist literature is completely useless. The best reference is probably Steven Hassan's books, in particular "Combating cult mind-control" (or one of the never). Order it on amazon today.