Girlfriend got insulted because she's flat-chested?

So, my girlfriend met my cousin for the first time and made fun of her for being flat chested and also made nasty comments about people her looking like a child and not hitting puberty yet. I was stunned by these comments and really annoyed about how he can say this

My girlfriend got afterwards really upset and started crying and is now really cautious about her appearance and everything because of it.

What should I do to make her feel better?

Updates
+1 y
Thanks for you're answers and the amount of detail in most of them.

So my girlfriend and I used to go running 2 times a week just a short drive away - we usually ran about 3km around the area.

After what has happened she doesn't feel confident doing that anymore after what happened - I've tried reassuring he but it isn't working - she's a better runner than me also and says she doesn't want to go anymore because she feels insecure about herself out in public.

Girlfriend got insulted because she's flat-chested?
Post Opinion