So I'm feeling unsure and a little worried... Help me?

So I really like this guy, and I've been crushing on him for almost a year now. He and I are also best friends... Yah, that makes it harder. It gets worse tho because he's crushing on my "friend" ( aka a back stabbing b!tch ). He was open to her about his feelings for her and she rejected him. To make things worse, she makes fun of his feelings for her, and text messages to her, and talks with his GUY FRIENDS about it. Just as a joke. Then she tells me about it as a joke and I don't really say anything when she does about it. But afterwards it makes me so mad and this is the part where I need help with. I'm not sure if I should tell him about what she is doing to him so he will stop trusting her with his secrets, and she won't have any way to keep secretly beating him up, but break his heart in the process of doing so? Or let it slide and just keep watching what she is doing to him and save his heartbreak for later? I would like your honest and sincere opinion on it if you would do that. Thank you!!!