If you know who guy is, and he’s on Facebook, you could play a few dirty little tricks IF you want him out of the picture.
You could set up a fake female profile and subtly catfish him... when he takes the bait, compile all his messages, screen print, then message them to your ex, and tell her he’s a low down, dirty cheating dawg.
If you do this, you gotta be very subtle... start by friending his friends, until you eventually friend him. Then you start off by just liking his pics, until he gets curious enough to inbox you. Also, by constantly liking / <3 hearting his pics / posts, your ex will likely start noticing, start getting suspicious of him, which will lead to arguments.
Make sure the chick who you use is sexier than your ex... it’s a no brainer he’ll eventually get baited.