Ask your friend how he or she felt about what they said or did.
Avoid interrupting. There may be a temptation to correct things they are saying but don't; just listen for now.
Always talk to your friend away from other people. You can't have a serious conversation about your relationship issues when other people are around.
If your friend won't answer or evades the issue, persist gently but don't push. It is important to avoid lapsing into a rant or an angry tirade against your friend, as this will only cause him or her to withdraw even further. Getting caught doing something negative to a friend is beyond embarrassing; it's mortifying and most people know it's an issue of broken trust.
Don't mention the person who told you. If they mention names, take your cue from there.
Bear in mind that people who are more removed from you than your friend may have an ax to grind or simply like to stir up trouble. It is important to keep an open mind before launching into accusing your friend of letting you down and spreading rumors about you. Consider what you know about the people who have fed the stories back to you and what their agenda might be. Consider also why you think your friend might have said something she or he shouldn't have––perhaps something slipped out without meaning to, perhaps a mistaken belief that someone else knew something caused your friend to elaborate or perhaps your friend was clueless about the real intentions of the person she or he spoke with.