I'm not sure even 15 quarts of oil would cause that level of damage. (The blown HG?)
Sure there is going to be some serious crank case pressure at times, but it would vent up out the cam covers, and into the intake. (Any signs of serious oil deposits in the intake? I can see quite a bit in the one intake photo, was it that wet throughout the intake?)
Looks like he blew the HG by overheating the car, then just added oil a few times in a vain attempt to stop it from steaming.... (People do really strange stuff.)
Any oil change place should at least check the dipstick before releasing the car, but like you mentioned, if the entire staff was new, or don't care, or know what they are doing, you can end up with some serious mistakes.
If he thinks the lube place screwed up, and you still have the oil and filter, the owner might be able to get the Lube place to pay for the rebuild. (Or at least offer him some money to resolve this problem.) It will not be the first engine replaced by a oil change shop. They screw up quite a bit. (I see a few insurance claims a year, or get referred to help out on them when claims of "sugar" in my oil, or water in the oil by unknown persons causing some vandalism to my car... (Covered under comprehensive that way.)
My only question is why did this guy let his car sit for so long after it stopped running? That is not what someone who had a faulty oil change would generally do. (The car ends up back at the shop, or towed in inside of a day, or week at the most.)