This is key. If I think about my daughters and the friends that they spend the night with, I know the parents and kids very well and it would be a non-issue for me. I also know my kids very well and know that they would feel very comfortable telling one of these friends' moms that they didn't want to take a shower with the other girls and wanted some privacy if that was how they felt.
This situation happened to the OP's friend's kid so we're getting it all third hand, but the OP said "and she (the daughter) just informed mom that the 'mom in charge' had her take a shower with two other gals". It's unclear to me (and maybe also to the OP) what was involved when the mom in charge "had her take a shower" with the other girls. Was that like, "you get in there right now and take a shower!" or was it more of a "time to take a shower now — do you want to all take a shower together?" Without more to go on I can't say how I would feel about this scenario, but I do know that I know the moms where my kids would spend the night and they absolutely wouldn't force the kids to shower together. If anybody came up with that idea it would be the kids.
BTW, moms whose kids have sleepovers, does showering usually go on? We've never done it at our house and my kids have never done it at their friends' houses. They have showered together at the pool many times.