One of the kids at my son's sleepover is a bedwetter, but I don't know which one. What should I do?

My 10 year old son wears underjams for his bedwetting problem. They disappear under his pajamas so sleepovers are no problem. Last week, he had 3 boys over for a sleepover. The next day as I was taking out the trash and dumping it into the dumpster I noticed a used diaper that someone had stuffed into the bottom of our garbage bag. It wasn't an underjam. It looked different. I looked closer and saw that it was one of those GoodNites. Obviously one of the other boys also has a bedwetting problem. What should I do? Should I tell my son? Maybe if they both knew about the other's problem, they'd feel better about their own bedwetting. Also, how would I go about finding out which kid it was?