I had this situation with my DD for a few years, she was in pull ups at night a lot longer than most kids. Here's what we did:
I put her pull up in a grocery bag so that it was hidden in her overnight bag. She knew to keep it in the bag, grab her pajamas, and go into the bathroom by herself to change. She put the grocery bag back in her overnight bag. We always made sure she wore a long t-shirt or gown with pajama pants underneath, so that if the shirt/gown rode up during the night, it didn't show the pull up.
In the morning, she knew to grab the grocery bag with her clothes to change into and again go to the bathroom to change. She put the pull up in the grocery bag and put it bag in her bag to throw away at home.
If the other kids asked why she wouldn't change with them, she just said she liked privacy or had to go to the bathroom. But honestly, no one ever asked her.
I love the idea of talking with the mother to let her know. My DD was only ok with this one time, and the mom was SUPER nice about it. Making it a very seamless operation! Most of the time, though, DD didn't want anyone to know, including the mom. So our system worked and nothing bad ever happened. I worried about the crinkle noise, but sleepovers are so noisy it was never a problem.
I will say, it helped to go through a little rehearsal of the process. Once her bag is packed, run through the steps so she can role play and just practice what she's supposed to do.
We did this probably 7 or 8 times until she was 'dry', and it worked every time. Hope your little one has fun!