I don't think I would, but sometimes life throws strange things at you. I would see them in a completely different light for a few seconds, but it depends on how they approached telling me, how long ago it was, how they had changed since then and how good a friend they are to me.
Although, since I only have a few really close friends. Probably not.
Depends if 'she' was hot..
I am 98% sure I wouldn't, that thought disgusts me, but this is one of those situations where, unless it happened, you never know what you would do. :/