the only thing i can think of is...
you have the aTV remote set to control the volume on the Bose via IR.
and he accidentally programmed "Bose Vol Up" to the play button on the aTV.
If you go into "Settings -> General -> Remotes"
it should have something about "Learn Remote"
I'm not at home right now, but hopefully there's something there about "unlearning remote" If not try going into learn again and see if there's an option to clear.
if that doesn't work, you can put a towel over the aTV, or place it inside a cabinet , as the siri remote is bluetooth
To set up volume control on your Bose, You want to use the instructions for "Program your remote for volume" on this page
https://support.apple.com/en-us/HT205225
the "learn remote" feature will let you use any remote to control your aTV.
so you could take an old DVD remote, and teach the aTV to respond to those codes.