BFF's fiancé was staring at my boobs. What should I do?

Kylie
Last night my best friend, her fiancé, my boyfriend, and I went out to dinner. While everything was fine at first, I noticed my boyfriend getting significantly quiet and kinda distant, while tightening his hold on my hand underneath the table. We all talked, ate, drank, and just when the bill came, I looked over at my friends fiancé and he was staring at my chest. He eventually looked away and my friend didn't realize--at least I don't think she did--that he was staring at my boobs. 
When we all left the restaurant, my boyfriend tells me that I had done nothing wrong, to not feel bad, and don't be sorry but the fiancé had been staring at my chest the WHOLE night. Not like a little, accidental glance but the zoning out, staring and getting fidgety type of looking. 
Obviously I don't know what to do. My best friend is super judgemental and territorial. I know that she hasn't had sex with him for a good amount of time and she doesn't let him watch porn. I don't know what I should do. Should I tell my friend? I'm worried that she will get upset with me (she can be a bit dramatic). However at the same time, they are engaged and I feel like he should be doing shit like this to this extent. I know that guys look but this was just ridiculousness. I'm talking like all night. 
Needless to say, it shouldn't matter what I was wearing but it was appropriate for a family restaurant and my boyfriend wasn't bothered by my top. I've know her fiancé for longer than my friend from school. I because best friends with my friend when we became roommates in end of 2012/beginning of 13. 
Ugh weird situation.