Help/advice needed

Me (33m) and my ex (30f) split up around 9 months ago it was and still is extremely hard for me to deal with as I thought she was my forever partner/sole mate.

I keep in regular contact with my ex's farther as I lost mine to Cancer around the same time he lost his son to suicide so I sort of became his son and started seeing him as my father I would also say he's a good friend.

Over the last few months here farther has become more and more worried about her saying something is not right that she has completely changed, he said she is angry sad and super argumentative. My ex is also type one diabetic and he said that she had been drinking so much that she started to lose feeling in one of her toes, but apparently she now only drinks on the weekends and the feeling has come back.

He keeps telling me that he's worried she is going to kill herself and starts crying.

I found out a few weeks ago that my ex is sleeping with someone that used to be friends with her family, this man is much older around 50 and extremely dodgy, he is a heavy methamphetamine user heavy drinker and is involved with gangs, not only that he ripped my ex farther in-law off 4years a substantial amount of money ripped me off and also verbally abused my ex so we all cut ties with him.

I'm extremely torn on what to do? knowing she is sleeping with this man after what he has done makes me feel awful/sick and like a huge stab in the back. I'm also worried about her and worried she is using meth as I can't see why she would be around him and this would explain her change in mood. I want to tell her farther but I don't want to get involved, I am also worried about what will happen if she or the guy she is seeing find out if I do say something.

Should I tell her farther or should I leave it? I would hate for something to happen to her and live with the weight of it for the rest of my life but I also don't want to come across as a crazy ex.

Really need advice as I have hardly been able to sleep thinking and ruminating over this.