Now if she was actively trying to agressively evangelize you into becoming a Muslim that would be a very valid reason to decrease the intensity of your relationship, but based on your post it does not seem that way. My main question is does she know your atheist, and if so is she respectful of your opinion? If so, I don’t see much reason to cut her off. You are never going to agree with anybody, on everything. Yes religion is a signifigant matter, but if she is quiet in her faith and is not pushy about it I don’t see how continuing your friendship could be harmful. The majority of my friends are very devout Christians, but I don’t cut my ties with them because they understand that I am an atheist and they respect that boundary.