Welcome Brandon and greetings.
The link given by ursamajor35126 was not for the " PC Charging System Troubleshooting Guide.doc" from Pogoman. The Files section of that forum is one level higher.
I suspect that a discharging battery has no other meaning than being a bad battery - unless something is using power. I hope that is not being the Parking position of the lock. Your father better takes the Parking fuse out. It is possible to remove the key in Parking position with the taillight still being out. I emptied my battery that way, only checking the rear lights and many more have had mishaps with it. This takes pulling the left air-duct and inspection cover out, eight fingers towards the back, and sliding the fuse-box off to eight a clock. The placement of fuses is only at the underside of its cover. When done and having inspected the contacts and fuses for corrosion then tie a small rope around the fusebox and cover, at the back under the clip. The fuse placement:
F Parking 10A B. (Brown wire) ('89 model only) "loose it!"
a Head 10A C. (Black-Red wire)
n Turn Tail-Stop Meters Dash Horn 15A D. (Black-Brown wire)
m Ignition Fuel 10A E. (Black wire)
A - A. (Blue-Black wire)
Back to the problem. When there is no additional device like an alarm and GPS attached or a short of some kind. then test the capacity of the battery. Having a low or surface charge the thing can be empty in no time. Test with a multimeter for 13 Volt. This level must not go down far and quick when you switch the lights on. Such damage may occur when the bike had been sitting for a while without charging, especially when it was cold. A possible cure is charge the battery up to a high level and discharge to clean the plates inside from sulphite a couple of times. A powerful charger must be applied, it should have 1 Amp max. (10% of the capacity) I use an adapter but that is unsafe and the process should be attended. Because chargers may be hard to get, I advice to do it like this: place a fuse of 1 amp or a little more in the plus cable. When the battery is shorted internally this precaution and feeling the temperature of both battery and adapter may save you from disaster. Also inspect the battery its surface everywhere for any signs of melting first. Just to make sure serious internal shortage is not being the problem here.
Next suspect is the charging system already mentioned. It may not be up to the task leaving the battery half charged. Your father will be in trouble riding when it is cold. Connect the multimeter and start the engine with charged battery. Before starting and after consuming some for the headlight, Voltage should be between 12,8 and 13,4. We see it go up to 14,2 when the engine is running and idles. Mostly it decreases to a much lower level when the engine is revved up high and the current gets regulated. When in doubt then do the charging tests mentioned what includes checking resistance and short-to-ground of the three yellow stator wires that plug into the regulator/rectifier above the fusebox.
Like StrayDog I replaced the battery but it not being the problem, I placed it back when it performed well after some good powerful charging (a Gel type has a larger capacity but is sooner exhausted and that did not help me in that situation). I rode about a year with LED bulbs and a light switch, the new Gel battery and cables with small clamps (still carry those) for jump-starting every now and then, with the sound of coughs and sneezing, in the trunk, home charging whenever needed and came into trouble riding when an unusual temperature drop in the weather caught me by surprise. Now, with my new stator, having to manually charge the battery or jump-start the bike seems unimaginable. This is still the same battery that I thought of being no good anymore.
One important note: there are three wires from three "coils" in the stator. Do check if they are all connected well on the regulator. Without corrosion and also the cable shoes in the plug being at the same level as the contacts in the regulator. Them sliding underneath will absolutely do no good. Bad contacts in any of those three can cause a demand way to high on the other windings. The stator may not only fail but be damaged on long Journeys.
Greetings. Alfred - The Netherlands. Don't you worry, I cloned my bike, being like several more, using a tripod and Photoshop in the Avatar picture above, even-though The Netherlands is a true PC800 affectionate county. We do expect some pictures from your fathers bike, when it is a keeper.