Tough decision to make... can you help?

I think you should keep a low acquaintance with Dee and stay close to Tom. Tom will have other girlfriends you can get close to but right now Tom needs you more than he knows. You have been best friends with him for too long to throw that away. I would say just call it a day with Dee and if she asks about why you haven't been keeping contact with her as much, say you just need to be there for Tom right now because he's been having a tough time. That's an awful situation you're in and I hope things work out for you.