Well sure she did. But she knew it wasn't just for her son. The girl was in her vehicle and the girl's mom told the girl to come home.
As the adult in the situation, she knew they were asking for something out of the norm--the two kids staying either overnight at the boy's house or at the hotel and she should have made sure they had the all clear from the girl's mom before proceeding.
DD will go to prom next year and unless something drastic happens, it will be her and her bf, her bff and the bff's bf. If I decided that it would be nice for the 4 of them to go the beach after prom, I wouldn't just rent a room or two and say "head on down after prom", I would actually talk to the other 3 parents involved. Not have the kids do it, ME actually talk to them. No way would I risk having my name as the one renting a room that someone else's kid would be staying in when the parent did not give the ok.
Since this wasn't something they discussed before prom, I wonder if the girl wasn't already drinking and the hotel room was the way for her mom not to find out.
ETA: Just went back and re-read to realize that it was the boy's mother who asked and the girl's mom said no. That makes it even worse, imo, the boy's mother knew without a doubt that the girl's mother said no. (I was thinking maybe the girl asked and just told the other mother that her mom said ok)