My best friend is cheating on her boyfriend. Help?

Anonymous
Okay so I'm in college and I'm rooming with my best friend. My ex-boyfriend had a best friend, Chad. So it was always us four hanging out. But my ex broke up with me so now it's my best friend, Melanie, me, and Chad. Melanie has a boyfriend in our home town but she's always talking about how he texts and calls too much. For a while now, Chad has been spending the night every day during the week and taking a shower and using my towels and wash rags. Melanie does the same because she's poor. Recently I've been noticing that I've been missing towels and wash rags. Also Chad told me that he's been having sex with Melanie since December and I didn't even know. I know this is none of my business. Melanie has her own reasons for not telling me. But this puts me in a bind. Her bf always messages me and asks if there's anything he should know and I've always told him no. But now I know that there is. I kinda feel like telling her bf. I mean my stuff is going missing (money and drinks also) but I don't want to jeopardize our friendship. But she lives with her bf on the weekends. I think she feels like she's stuck and has nowhere else to go so she's still with him. Also, my other friend told me that Melanie was snapchatting this guy and sent him a pic of her boobs. Her old ex told me that she's addicted to sex. So should I tell both Chad and her bf about what I know? Chad talked to me about it and a lot of what he told me cleared things up so I'm pretty sure he's telling the truth. I really don't know what to do. Help?
My best friend is cheating on her boyfriend. Help?
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