>>97Assuming the goal is to simply to get the trainer to quit being an asshole, the best route might be to compliment and validate him.
In short, it's extremely hard to treat someone like shit when they are complimenting you. A sort of tactical surrender often lulls bullies, who find less amusement when their victim isn't resisting, fighting back, or getting upset.
'Youre right, I wish I had the skills you had, how can I do better' (said somewhat genuinely) can be quite disarming to an overbearing bully.
If you want to get the guy fired, it would probably be best to document to abuse and take it to HR. That may get the guy fired, but it might not actually help your friend in the company, who might be seen by other trainers/managers as a snitch.