What Moray said. It was no big deal in the sense that it was an accident; the kind of thing that could happen to anyone. There is no need to assume evil intent on his part - in fact, I think it's pretty rude to jump to a conclusion that he's some sort of pervert based on this accident. Still, in our society, we cover our genitals in public, so in order to save embarassment for him and onlookers, the decent thing to do to bring it to his attention so he can make adjustments. No. Big. Deal.
This sort of thing really can happen to anyone. Back in the 1990's, we used to wear romper dresses -- short sundresses with very full shorts instead of skirts. I was at a party in an outdoor bar and one of my romper legs got flipped up in the back. I was wearing thong underwear, so one of my buttcheeks was fully exposed. Another partygoer whispered to me, "Hey, flip your dress down -- your @&& is hanging out." I was glad that someone brought it to my attention so I could fix the problem before I flashed the entire room.