It sounds like there should be something in the game's system that detects when a mentor abandons the fight immediately? Seems like an easy way to detect 'bad mentors' who aren't interested in sticking around and helping people, if they have a constant pattern of doing it.

(I'm a long way off Lv70 anything, but I'd like to be a mentor when I eventually get there. I probably wouldn't be the best expert on tactics, but I'd do my best to help people. Also the 'watering can' icon sounds like an excellent idea.)