Ok lets break this down into managable steps-

three friends:

friend 1= you
Friend 2 = friend that revieled
friend 3= friend with the problem that # 2 came to you about duing the 5 minute chat.

Are you saying

#3 had a problem with you (#1) and went to (#2).

#2 during an innocent talk accidentally told you that #3 was talking about you and a problem with you.

Now you have questions -

Why did #3 talk to #2 about you and the problem that she had with you instead of telling you there was a problem?

What to do to fix the problem?

If I am getting this right first you need to:

sit down with 2.

2 is probably kicking themself in the tush for open mouth and insert foot accidentally and is waiting for the fallout.

Tell him/her thank you for letting you know there was a problem, and you appreciate it, and you understand it was told accidentally and you are going to work it out without revieling the accidental disclosure. The problem is between 3 and you so you are going to keep it that way. ask her if she can do you a favor of If the person comes to her again about situations involving you ask that she tell that friend to discuss the problem with you that way she doesn't get put in this situation again. three people can be friends without having to worry about telling on each other or fighting each others battles accidentally like this.

Then you need to sit down with 3. Let her know that when you care alot for your friends sometimes you like to check in with them to make sure everything is ok with the friendship. Sometimes problems happen where one person is afraid to talk about the problems during the friendship, so you like to take the time with your friends to let them know how much you care for them and that you are there for them no matter what be it a problem in thier lives or a problem with the friendship itself. So tell me how do you think things are going here....

That will open the discussion of your friendship and possible problems she may have with you that you aren't picking up on and she is going to others about, without you having to reviel you know about the problem already.

when she does disclose the problems remain calm. sometimes hearing things about our self is not fun, but it is helpful and how problems get solved in friendships.