insomniachollie wrote:
It's one thing to go to a manager and say "this person has most definitely done X to me and it's affecting the work environment." You're lodging a grievance against another employee about something which directly happened to you. If it happened to somebody else too then that's just additional evidence.

It's another to listen to a shit stirrer like that talk shit about the manager, have no grounds on which to believe that it's true other than the word of the office shit stirrer, and then go confront the manager as if the accusation has any grounds. Especially if it's a serious accusation. You could harm your relationship with them, possibly harm their reputation, start shit with the co-worker you tattled on... and all for an as yet unfounded rumour. Not worth it. 
I agree with all this. If this friend does or says anything racist around you then you can say something to them but until then I'd try to stay out of it. You don't want to cause any drama at work if you can help it. Until you've witnessed it yourself it's still a rumor but if it does happen around you just make it clear you aren't tolerant of that type of behavior.