DD is 5.5yo and in reception equivalent. A few weeks ago she was very upset when I got out her new puffy winter coat and said she "didn't want to look fat" (for context, she's smack on the 50 percentile for weight and height). After some gentle chats with DD it emerged that she doesn't think she looks fat but was afraid someone would say she looked fat in the coat. I spoke to her teacher who was shocked but great, and has done some very general work around size and shapes in all contexts, etc, etc. Around the same time, DD mentioned a boy in her class who was teasing her about a nick name she has always called her best friend, also in the same class. Today, DD mentioned this boy said she looked fat in her coat. 
Thing is, the boy in question is a lovely child. He was in the same preschool as DD and her bestie, and I know his mum quite well. He's had some trouble settling into school and I think is trying to find his place by being a joker. DD is very sensitive so tends to take this kind of thing on board, where a child like ds would barely notice it.
I'm not sure if I should tell her teacher that this boy has said something specific to DD. Any thoughts? My biggest concern is that he's saying more than this isolated comment and she won't tell me as she doesn't want to get him in trouble, but I don't want to be making a mountain out of a molehill either.