I don't have hard and fast rules about this anymore. If I had a friend who treated her guy like crap, and they'd been broken up a while, I might consider it. I don't like people who treat others poorly, so it might not matter so much if I lost her as a friend, which might play into my decision process. I have scaled down my number of friends over the years, and cut out many acquaintances, so these days this isn't a likely scenario for me, but I did date an ex-fiance's friend years ago, as well as his (the ex's) brother. As you can imagine, both situations were difficult.