to add to the above: either your father can go ahead and get his retina fixed by a retinal surgeon, who can remove the lens via pars plana lensectomy, and thus leave your father aphakic, until the retina is stable enough, then he can get another surgery, by the refractive (cataract) surgeon to implant an IOL. Or he can go ahead and get cataract surgery by a cataract surgeon, who will implant an IOL (thus your father will be pseudophakic), then immediately after your father's retinal surgeon can repair the retina. The above are all assuming that his cataract really is impeding the surgeon's view. Neither method is "wrong".
getting a cataract removed before retinal detachment surgery should not be a problem, but ideally, it should be done on the same trip to the operating room since retinal detachments need urgent care and it saves trips to the O.R. It is true that a cataract can make it hard for the retinal surgeon to see the back of the eye. The reason why the nurse said that most people get cataract surgery after retinal surgery is because cataract development is inevitable afterwards in phakic patients and they generally don't have cataracts before retinal surgery or it's in the early stages.