A friend of mine has a huge bite mark on her, just under her left breast. Is this abuse?

We were mucking about and we're tickling each other and I accidentally touch just under her boob and she flinched and a tear came to her eye. I said sorry did that hurt you and she said, it's nothing. I managed to get her to mention it to me and she showed me this massive bruise. It had human teeth marks in it and she made me promise not to tell anyone but I don't know what to do. She started telling me stuff about this man she knows who's quite a bit older who is getting drunk all the time and starts touching her in places and he even said he wants her to have his kids. He apparently pinned her down and made her kiss him while he was touching her down below part and she said to him get off but he didn't until she kissed him. She said to me that he's an alright guy when he's sober. But he's always round her house all the time. She's 16 and he's 30. He always says he's sorry to her but he keeps on doing it. What do I do?? Should I say something, she made me promise not to tell anyone but I'm not sure if I should keep this secret or not. What do you think??