Well, they would know what kind of person I am and what I believe in so in telling me, they would have a specific reason to tell me. Otherwise, Unless instructed by the holy spirit, I would still continue to be their best friend but pray without ceasing, and not doing that horrible christian thing of throwing every verse that they can find to shame them like they aren't people with feelings. I would continue to show love, inviting them to church youth nights, and continue to be their friend because in a world where gay people are given a hard time and where atheist are basically given the scarlet letter in the bible belt or religious area, they are going to need some support from a friend more now than ever. I don't support either but I hate when people are treated harshly for being different, many people have stories that makes them like this, not just because they wanted to and usually people of the opposite view prove them right and further destroy any attempt to getting them back.
That's just me.