Solo diving is another discussion altogether. 6 in one half a dozen in the other. As a diver that embraces the DIR philosophy of diving, I believe team diving contributes to an extremely elevated level of safety and pure enjoyment. I do, however dive with a gentleman who dove for many years as a Navy ordinance disposal diver and believes he is better off solo. In comparison to diffusing sea mines, I think a casual romp in the quarry will be manageable for him. As for the swim test...TSandM hit the nail on the head. The test has more to do with measuring your level of cardiovascular fitness rather than your competency as a swimmer. Sudden cardiac emergencies are not uncommon and the worst place to have one is under water. You become a danger to yourself and your buddy. Divers have an obligation to themselves and their dive buddies to stay in reasonable physical shape. Does this mean you have to go out and run a marathon tomorrow, no. You just have to make an effort to condition your body to operate under strain. Remember it's not just the activity in the water, you are carrying dive gear to and from your vehicle, sometimes traversing across sketchy entrances, battling current, swimming against large swells to make it back to the dive boat etc... Poor fitness can also lead to an increased likelihood of getting bent. If you make the jump to rebreather, poor cardiovascular fitness can lead to death on the unit from labored breathing.
Bottom line, it's an important part of the certification process. I can teach someone how to assemble gear and breathe from a reg and achieve trim and buoyancy. What I can't teach is fitness. You have to do that yourself. I hope your father will continue with his scuba training. I used to DM for a local shop and had a couple of people have difficulty with their swim test. As a former swim instructor, I was able to help them improve their technique a bit and they found the swim a lot more manageable. One common mistake that a lot of non swimmers do is attempt to "bicycle kick" when they swim. A proper flutter kick will greatly assist in the efficiency of the stroke. Have him practice on a chair with his legs straight out and feet flexed with toes pointed. The kick should originate from the hip and glutes. This will help him keep his body in a streamline position and make the stroke immensely easier to do.