What would you do in this situation?

A friend of mine recently broke up with a girl he was dating for over 6 months. A couple weeks ago me and another friend of his were out clubbing and saw his girlfriend grinding up against another guy. We took notice and texted Tom, telling him his girlfriend was out grinding with some dude. He said she couldn't have been there as she told him she was visiting a friend in the hospital. So the next week, a couple friends of ours saw her leaving a club with a guy, she didn't see us even though we passed right by her. So a couple days ago Tom broke up with his girlfriend Jan, because she was cheating on him. She of course denied it. Since then we have been hearing weird and dirty things about what she has done, what she was like in highschool. Apparently she cheated on Tom with over 8 guys, that we know of, were sure its alot more. Tom had recently bought tickets to go to Europe in March for the two of them, he made reservations and spent alot of money preparing for the trip. He was devastated, but thankfully has moved on since. Now Jan keeps texting us all, blaming us for ruining their relationship. She keeps pestering Tom. She has even come on here ranting about different stuff, messaging me and another girl who comes on here. Its like shes stalking us.

What should we do? She threatened to "ruin our lives" She hasn't done anything violent to us, yet. But we know she has a drug problem. It's quite concerning, we don't know who shes connected to or what. Any advice?

Thanks anyone, this is really concerning and quite strange.