Heard Girlfriend of brother in law is playing away.

I have been told through two good friends that my brother-in-law's girlfriend of 6 years has been seeing her ex boyfriend (P).
This ex boyfriend was a friend of my brother in law (M)until he started meeting up with his ex girlfriend. The ex boyfriend fell out with my brother in law and they have never made up.
I was first told from my best friend when she was shouted on from the ex boyfriend (P), she stopped to talk but he was drunk and was boasting he was going up the road with my brother-in-law's 'burd'. She saw her from over the road but could not confirm if it was her or not. She with held telling me because she wasnt sure he was saying it because he knew she would tell me to stir trouble.
Then at a birthday party my other friend was speaking to ex boyfriend (P) were he told her that the girlfriend was coming up to his house but didnt say for how long this was going on for. He said he told her to f*** off when she first came chapping because he didnt want anything to do with her.
Now his brother, my partner, and I know and dont know what to do, because we know it would cause devastation and not sure how he would deal with it.
Please tell me what is the best thing to do, would it be best writing him a letter, telling all first?
Thanks