Cheater, Cheater Pumpkin Eater?

I am faced with a challenge and I am not sure how to proceed. It is one of those situations that either way I precede someone will most likely get hurt. I have been sitting here for a few days trying to figure out which road to take but I still haven't managed to settle on one path. Here's the situation (I will leave out details to protect the people involved): I received a Facebook message from a stranger. Typically, I just ignore messages like this but it got my attention because the first line said they were trying to contact a relative of mine. I decided to go ahead and read the email. BIG MISTAKE. The person went on to say they were trying to reach my relative because they had slept with this relative’s spouse. They claim to have not known up front this person was married but now that they found out they wanted my relative to know that their spouse is a cheater.

My first thought was "I don't want to know this information. I don't want to be involved" but this can't happen since I read the email. My second thought was "Maybe this person is crazy and just trying to start trouble". I can't imagine this person doing this to my relative but then again we hear stories like this all the time. So I decided to proceed with texting my relative's spouse and asking if they knew this person. They responded that they did. I then went on to tell them that I received a message from them with a picture of the two of them smiling together. They then responded to me asking me not to say anything to my relative because they have been going through some hard times and this would be very bad. They went on to say this person was crazy and stalking them. They said they told this person they were married. They denied sleeping with this person but I realized something, whether emotionally or physically, had happened otherwise they wouldn't be trying to hide it. I wanted to clarify some things with this person so I called to talk with them. After a 20 minute phone call I was still left with doubts and they still wanted me not to say anything. I told them I would think about it.

That's where I am now. I know I should say something to my relative. I mean they are family and I would want to know if something happened between my spouse and someone else but what if nothing happened. They say nothing happened but then why are they trying to hid it from their spouse? Why can't we just tell my relative that some crazy women contacted me and they should be careful? I know I should tell my relative but I don't want to destroy this marriage. I don't want to tell them out of selfness of wanting to avoid this situation. I want to trust this person but I just have an uneasy feeling about this situation. I don't know how to proceed. This person suggested that I don't tell my relative and if they find out they will either tell them that they begged me not to say anything or leave me out of it but can I live with that? Can I see these two and pretend I don't know this information. My relative and I are close; I don't want to destroy that.

What do I do? Are we looking at a stalker or a cheater, cheater pumpkin eater?