My Best Friend's BoyFriend is a Cheater!?

Let's call my Best Friend Tasha, and her Boy Friend Jack, who has a man-ho reputation. So, I was talking to another one of my close friends (let's call her Hanna). She used to go to our school but she moved. Apparently she likes this guy, whom happens to be Jack. The catch? They've been texting eachother and all that what not and he told Hanna that he doesn't have a Girl Friend!! Which so happens he does! Upon mentioning his girl friends name in front of Hanna at the the pool, he shoved me into the pool saying, "Time to go for a swim," and walked away. What should I do! Because, Hanna wanted me to tell Tasha 'bout cause she felt horrible and now Tasha's ticked off at me!! She thinks we're lying but we're not :( We need some help ASAP!