Be glad that your daughter told yhou about the intended plans. She is asking for your help - Her answer to friends will be 'no I can't spend the night. My mom set the rules no sleepovers with boys.' You become the bad person. Which is OK. My husband and I have always been bad. Our 2 daughters have turned out pretty good we think. I wish I had a copy of the poem title the "The Meanest Parents" it give you lots of encouragement.
Don't let her spend the night. The other set of parents are setting themselves up for real trouble.
Sorry to on going, but we need to keep morals alive. Life is hard enough. Remember you will be in my prayers.