I'm confused. what did he do wrong?

Somebody has emotional dysregulation.

She must be fun at parties

What did he do wrong? He decided to play with this psycho.

Playing a sport that involves hitting balls at speed at each other, and she's mad about getting hit, she should stick to checkers.

She’s probably just stressed out because, for some completely unknown reason, she has never in her life been invited anywhere a second time.

Mainly what he did wrong was playing with an emotionally unstable narcissist.

For whatever reason tennis really brings the asshole in people.

There is no rule prohibiting a player from aiming directly at another player; it is treated the same as targeting open space, and 'jamming' a player by aiming at their body is a valid tactic to make it difficult for them to swing. In doubles, when a net player is close to the net, passing them around the sides is often difficult anyway. If you hit another player, standard etiquette is to put a hand up in apology, which this guy did. The only thing that's really poor sportsmanship is aiming for the head, but this guy's shot was at hip level.

Such a fucking diva, the ball didn't even hit her that hard, I could understand it a bit better if he hit it at full force at her. 

I'd hate to play Monopoly with her.