I am 34 years old and I have a friend I've known since I was 4. She always ask me to find out if her guy is cheating or what, and when I show her solid proof that he is she gets mad at me... Her last guy she was messing around with is one of my uncle's best friends who is like an uncle to me, so she wanted me to get some dirt on him and I wouldn't get involved because I know she would have stop talking to me again, so finally I had to tell her that he was getting married once his divorce came through, now she didn't know that he was still married but this I did think she knew, but she didn't know he was going to re-marry, she was talking about getting his name tattooed on her and I wouldn't let her go that far with it, so once again she stopped talking to me about 3 months ago, now she hasn't heard or seen from him in awhile and she contacted me to find out if I talked to my uncle to see what he's been doing, should I not talk to her or talk to her but not about him?

Never get involved in the stuff she is asking.
Tell her that what your uncles friend does is none of my business and I don't care.
If she can get mad and not talk to you for months then she is no friend.
Friends are friends through thick and thin; not playing petty spiteful silent treatment games to punish you.

Then your uncle realizes what you are up to and feels he can't trust you so he quits talking to you, N0T worth it!

It sounds like she is only using you, and the silent treatment she is giving you is just plain immature, she needs to GROW UP! And start treating you with respect! Im glad you told her the truth about your uncle and besides she is old enough to stand on her own two feet and sort out her own problems. You can be there as a friend but she has to learn from her mistakes and deal with them. As the saying goes FRIENDS come and go but family is always by your side. You seem like a trustworthy friend and you deserve to be treated right. Goodluck with everything, don't let your so called mate bring you down and just ignore her if she keeps giving you the silent treatment, she will soon realise how much your friendship means to her.