The red, or now silver cap 15W-50 was/is a good oil, although not really as good as the bike 20W-50, which is more shear stable and has higher zinc and phosphorous levels for potentially better wear protection in high heat/high stress conditions. But for the cost savings, many folks use it, and with good results.
Currently I’m using HD Syn3, but for my next OCI I’m going the M1 15w-50 route so I have managed to grab a couple of jugs, because here in Australia the M1 15w-50 is fast disappearing off the shelves and being restocked with M1 5w-50, which is also selling for less and meets some Porsche & Benz specs.
It is a Road King. I think it is a 2005 model. When he takes long trips he has to travel with oil.
Tell your friend he has defective valve guide seals. It was a warranty item back in 2006. Most all of the TC88 motors built in 2005 and some built in 2006 had the Harley -02 valve guide seal that caused excessive oil consumption. Many guys got the seals replaced under warranty. He needs to get seal # -02A installed or get a custom seal kit from Big Boyz Cycles. Look here for a discussion. With links to photos.
There is nothing wrong with your friends engine as once he gets the seals replaced it will stop burning oil. If he bought the biuke new and brought it back for oil consumption then the dealer & HD owe him a warranty fix. Some of the lazy dealers and the MoCo were telling people 1 quart in 1,000 miles was "normal". Which is total horsehockey. My 06 TC88 uses about 4 oz of oil in 5000 miles.
Plus some folks actually do not know the correct procedure to check their oil because all bikes are different. (Some you check on the kick stand, some standing upright, etc.) Lots of people overfill the bike and thus it burns the oil out or dumps it into the air cleaner.