My girlfriend was intentionally accidentally exposed? What should I say or do?

me, my girlfriend , her friend and boyfriend went away to a shack for the weekend a few weeks ago. They also brought along 2 of there guy friends who I knew but not well. It was night and we had sleeping bags layed out on the floor to watch a movie. My girlfriend was laying on the floorwearing a tight black tank top that had two buttons on the front. She was not wearing a bra underneath because she has medium perky boobs and while standing up it would cover her fully but while laying on her side showed some cleavage. She had the remote and we couldn't decide on what to watch so me and 1 of the random guys went out for a smoke but could still see into the lounge from an angle. My mate asked for the remote which she said no to so he tried to grab her arm and take it. They started play fighting and she was laughing. My mate grabbed her in a bear type grip from behind with his arm across her shoulders. This caused my girlfriend right boob to show cleavage. She lifted her arm with the remote up so he could not get it. I was seeing this happen from outside. When she lifted her arm because my mate had his arm around her it caused her top to pull down just barely covering the top of her nipple. I was preying her top would not slip down, but my mate new any slight movement with his arm would cause it to when she moved. knowing this he purposely pulled her arm down while his other arm pulled pressure on her top.. I know it was on purpose because he was smiling while doing it and could of easily grabbed the remote. She was giggling and still holding the remote not realising what he was doing until her right nipple started to poke out. I couldn't believe what I was seeing but he kept pulling until her full nipple was showing. His girlfriend saw this and instead of telling her to fix her top she whispered into the other guys ear and pointed to it while giggling. He then tugged gently on her top exposing her right boob? My girlfriend had no idea. They covered her before I came back in. What Should I do/say?
My girlfriend was intentionally accidentally exposed? What should I say or do?
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