What would you do?

This is a long one y'all. I appreciate any feedback you can offer. I am posting anon, because I know she is also on this app too.

I am really struggling with what I should do in this situation. I am torn between what I feel is right morally & my obligation to be a loyal friend.

Here is what is going on:

I have this friend who has been with her boyfriend for almost 8 years & they have a child together. Her boyfriend is good friends with mine (that is how her and I met.)

She has been sexting with this guy (an old coworker of hers.)

The first time (that I know of) was about a year ago.

There was a little back and forth & then it was over. She never talked about it again and I am pretty sure she stopped talking to the guy (at least in that way) for a while.

Recently, she brought it up again. She read me some things he said to her (sexual) and told me she has sent him pictures. This was about a month and a half ago & until the other night I thought it was an isolated incident.

Well, come to find out they pretty much have been talking here and there the whole time. She told me he sent her a sexual video & sent me a screen shot of a sext he sent her. I am sure I am not getting the full story from her as she says things like "idk what to say back to that." But, if she wasnt instigating as much as he is I dont think it would be going on as long as it has.

Anyways, she is my friend and has told me these things because she trusts me with them.

However, I feel awful. Like I am the one lying. I am so against cheating in any way & I feel this has completely crossed a line. I know if the tables were turned & it was her boyfriend doing this, she would be crushed.

I feel like he needs to know, but I am not sure it is my place.

What would you do?