I'm sure someone else has probably mentioned this by now, but just in case, the fact he's dry at yours suggests that he's just not sleeping as deeply at yours.
Vast majority of kids who wet the bed do so because they're deep sleepers and their brain isn't picking up on messages from the bladder during the night. If they're sleeping more lightly, which is common when in other beds/houses/places, then they'll wake up easier if their bladder sends the message to the brain that it's full.
My DS is 9 and still wets the bed. Has been under the continence clinic for a year or so now. He's been on sleepovers at my mum's and school residentials and not wet the bed, simply due to later bedtimes, and then sleeping lighter when there.
At 5 he's probably too young for a referral to a continence clinc, it's 8 where we are, but it's worth suggesting to your daughter. Plenty of things they'll try with him - medication to help produce the necessary hormone, alarms, checking he's not constipated and medication for that if he is... But 5 is still really young and well within the normal range for bed wetting.
More water in the day time will be suggested first, so that's something your daughter can do without any Dr's input. This will increase his bladder capacity which might help him hold more wee in his bladder overnight. But then a set time for last water before bed. My DS has last water 1.5 hours before bedtime. I'd certainly do away with the overnight sippy cup.
It's absolutely exhausting dealing with it every night and then all the extra washing in the morning. I can understand why your daughter feels the need to vent...the same as you're venting here...
But trust me, the fact he's dry on the ocassional night at yours does not mean that your daughter is doing anything wrong and you're doing something right. Please don't take that attitude with her. Hope it resolves itself soon, it does for many kids.