You don’t have to tip the waitress anymore than you tip the plumber. Just pay the bill and leave. And you’re completely free to tip the plumber if you want. However, the traditional compensation system for waitresses has the tradition of tips built in.
Without tips, the waitress makes almost nothing, and what you’re really paying for with that tip is supposed to be the nature of the service she provided, whereas with a plumber, he can be a jerk or a nice guy and you don’t care nearly as much as whether he fixed your pipes.
A good plumber is doing more, and can do far more, than any waitress you ever met, especially when he goes far beyond what many think a job should require, the kind of thing that shows up in plumbing work.
Tell the restaurant owner to pay his own employees and leave you out of it.