Perhaps cultural difference but what exactly is wrong with that if the girl gave someone permission and I don't mean inferred permission but actual spoken permission? In this case, the girl may have been grinding on him and if she was wearing a short dress, it would rise up especially with the way some ladies like to twerk and voila, exposed under dress.
For me it is key to know the circumstances because having gone clubbing in the UK and having had girls desire to dance with me in a very sensual sexual manner ... I am actually interested in knowing what happened. If it is that he put his hand up a dress with no permission then throw the book at him.
PS if the thought of a man putting his hands on a lady's undies offends you - please stay away from West Indian and West African parties of this generation - I tend to want to look for holy water and start spraying the entire place to get rid of all the horny peeps there. The way they dance nowadays scares me.
PS shout out in memory of my female minder when I was in university. She ensured that I never danced with drunk ladies in my first year. Thanks to her - the moment I am in a club and smell alcohol on a lady, I am away from that area.