She lived nearby so I would go to her house, and he was often there.
I then realised she seemed to slap his butt every time she told him to do something, almost like a non verbal "go, get it done"
So anyways the really funny thing was when she accidentally patted my bum on reflex, and got embarassed when she realised what she did and apologised but I just shrugged it off, mostly because I thought it was funny.
From where we usually sat you couldn't see inside the house, but one time we sat in a different orientation and my chair looked into a room.
The door was closed but midway through she went and opened it to go inside, and lo and behold my friend was lying on the bed watching some show in his undies.... If only he was female
Regardless it was funny to see, and he didn't even realise it as he already began taking them off as his mom was leaving (I think he thought she left but she slowed down) and effectively mooned us
Unfortunately there was no way to tease him about it because he left for a trip the next day and returned two weeks later, by which the teasing would have been stale.
I also noted that he effectively stripped naked despite his mother's presence, so basically informing me that I wasn't the only 15 year old whose mother saw him naked.