It's interesting the way we're all taking the bare outline of events presented in the OP and imagining the situation in different ways, and it's alarming to me that so many people are imagining worst-case scenarios. What was described was not "the parent telling the kids to strip and then looking at them."
I've got to agree with that. The OP's child wasn't even involved. She heard about this from the child's mom, who heard about it from the child, so we're hearing about it at least third hand. We really don't know exactly what happened nor how it was suggested. I think to demonize this sleep-over mom and say a call to CPS is warranted is over-stepping bounds. It's possible that something nefarious is going on, but it's just as possible that it was completely innocent.
My 8 year old absolutely loves going in the bathroom with her friends. I have no idea why. And they love to get into mischief in the bathroom. I could easily see a scenario where one of my dd's friends might suggest getting in a shower (and there's plenty of room in a tub/shower combo for 3 kids — my 2 shower together often enough, and I used to shower with them when they were littler and I'm bigger than the two of them put together) and the sleep-over mom would agree and then knowing what mischief my kids like to get into in the bathroom (writing on the mirrors with soap—and that's my 11 yr old—splashing around, and generally cutting up) I would expect the mom to poke her head in from time to time to make sure her bathroom wasn't being destroyed and everything was okay.
I mean, even if there wasn't nakedness and a running shower involved, if you had three kids over at your house and you heard lots of squealing and splashing and giggling going on in the bathroom, wouldn't you want to check to see what was going on?