That would depend entirely on the importance Rainmaker places the social aspect of his life against that of his professional one. I would be more inclined to confront someone messing about with my life outside of work than within it as I place more value on my relationships outside of work than that of my employment. I can always get another job.
We also have to consider the importance Rainmakers friend puts on his friendship, if he cannot or will not support Rainmaker in clearing his name and in doing so effectively supporting the gossip mongers I would question the level to which I would place that kind of friendship.
I will admit I am a little confused over what concerns Rainmakers friend has exactly, as this rumour could have gotten to Rainmaker from any number of sources as the very nature of gossip means there will be more than a single person who has heard it, and in any event, it is untrue and malicious so he has no moral obligation to honour a confidence that requires him to compromise his ethics or integrity.
It seems to me that some people in this thread are more interested in what Rainmaker is doing, rather than what he is asking. If it really is as important as you think, then all that needs be said is whether it is a professional or social organisation, what that organisation does or the specific nature of it, is immaterial.