Honestly, I'm willing to play with it to whatever extent you're willing to go along with it; obviously it'd be godmoding to say "well of course he can overpower even the most loyal of blah blah blah etc etc", but if you're willing and open to something like that, I'm more than willing (and he's more than capable, really) to say that he somehow managed to get inside the 'mon's head and hit the right number of buttons to make them turn on their trainer.

It'd probably be easier still if it were one he'd seen before and spent some time around (I.E. letting him get familiar with how its brain works in advance), but I can easily work that out on my end if you've got a 'mon (or a couple) in mind that you'd like to see it happen to.