I'm in a bit of a predicament and I was hoping this community could help me. Forgive any spelling and grammar mistakes. I am dyslexic.

I (18f) am in a small group chat of long time friends. We've been friends since middle school and have all gotten along very well until now. Two of the people in our friend group are dating. Bella (17f) and Asher(18f) (not their real names ofc). they've been dating for two years now. And as far as I knew there relationship was going fine. For context to this story, Bella is slightly overweight. She's struggled with her self image for a long time. And we've all done our best to help her through her weight loss journey. Asher has been one of the people closest to her and helped her the most. So what she said on the phone the other day shocked me. Asher and I were on the phone the other night discussing our plans for prom. It's our senior year and we want to go all out this prom. The other girls in the group chat have already picked out their dresses and me and Asher plan to go in suits. I had brought up Bella's dress to Asher because I thought it was really cute and I was excited to see her in it. But I noticed Asher had gotten really quiet when I brought it up. I asked her what's wrong and she Hesitated for a moment before saying. "I don't know... Sure it's cute and all, but I just don't think Bella would fit into it nicely.. You know she's a little-" to say I was floored was an understatement. I can't imagine speaking about a friends body like that. Much less my girlfriend. In a state of panic I immediately made up and excuse to end the call and called Bella. I told her everything that Asher had said and it wasn't hard to imagine that she was devastated. She broke down into tears on the phone and I did what I could to comfort her. She was thankful that I told her but said she needed some time to process. To make a long story short she broke up with Asher that night. And now Asher is pissed at me for telling Bella about what she said. Some of the girls in the group chat agree that I did the right thing. While others think I should have kept my mouth shut and let the couple deal with this themselves. Personally I don't think I did anything wrong. If I was Bella I would want to know to. But I could use and outside opinion. So AITA for telling my best friend what her girlfriend said about her?