In a casual/meaningless match, I would never hit at the net person's head on a sitter. Particularly if the net person is a girl. If I did hit at them, it would be a dipping heavy topspin shot because I don't want to clobber someone.
If your partner's second serve is a joke, play back on second serves for your own safety. It isn't worth getting a detached retina for a meaningless match.
However, in a competitive 4.5+ match, all guys, all bets are off and it is kill or be killed.