I worry no-one would believe John could get that bad.

I think you'd have to direct it the right way--suicide runs rather than cutting or starving himself or anything like that. But yeah, you would have a job explaining how he managed to survive losing so many other people without falling apart, and why he's suddenly unable to cope with loss...especially in that he's losing someone he only sees every few months. I admit, that will probably be a challenge. I don't blame you for hesitating.