HELP: SHOULD I TELL HER BOY FRIEND SHE IS CHEATING ON HIM WITH ME

Somebody sent me this email asking for advice on whether to tell a guy his girlfriend is cheating on him, please help him out and please be sincere, will check back later to read you advice and probably give him mine. read email below

Sisters and wives pls I need ur advise...sincere ones pls..
Months ago I met this girl online and we started chatting. She sent me her number and I called her a few times. She stopped chatting with me then she started again. In a bid to know her more cos she sounded really nice I checked out her friends list. Found that I know someone who knows her. I found out she is dating a married man and his wife is close to people I know. This man abandoned his wife and kids in Europe for nearly 4years. All the while he was seeing this girl and treating her very nice and splashing a lot on her. I asked her about it and she said he was not married but he has kids. I showed her proof he was married I don't know what she did with the info though. She was chatting like she was into me and even asked me for money. I really wanted to send it to her but I changed my mind last minute. When we talked on the phone we did the things that ppl do these days on the phone. I wanted her to come here to canada and she agreed. Meanwhile this man thinks she is so good. I have found she pretends to him a lot. When I found out more things I decided not to chat with her anymore. I told her so. I recently found out that this man and the girl were planning to get married. I know he is not divorced from his wife. I was so angry that a man could do that to his family. I don't why it's affecting me like this. I feel for the man's wife. I threatened the girl that I will forward the conversations we had to the man she said no problem but I haven't. I need ur advise on whether I should or not. I know he is not in his right senses. He needs someone to wake him up. Should I forward our conversations to the man? or I should not get myself involved. God bless you