"One of his programmers in particular--who is currently his most-senior on-staff programmer--is bad." If you have a good relationship with your client make you

"One of his programmers in particular--who is currently his most-senior on-staff programmer--is bad." If you have a good relationship with your client make your concerns known and the consequences (price). A good client will take this in and may give you some insight. Give yourself a set time-frame or project. Wait, see what happens. If nothing happens, increase your prices adjusting for the losses and get a new client.