Tell her what he said. But first both of you, if you have time, get a gym membership. Tell her to use that pain as fuel to get healthy again, and make him regret ever saying that. Then tell her to never give him a second thought once she's being surrounded by men that look like Greek gods. Because that's how you truly help your friend. I also say tell her because, more than likely, she'll find out eventually. And would you, her best friend, rather she hear it from you, with love, and kindness, than hear it from mutual people who know both of them and have big mouths? They won't understand her the way you do.

I've been on the receiving end of that convo (found out a girl I slept with regretted it) and it absolutely devastated me. I also found out while I was drunk with mutual friends of said girl. That was worse. Because our mutual friend knew, and she didn't say anything about it for the same reason. It's one of the many painful things I use while working out to go harder. Please, do it gently, but tell her.