You don't want to be 'large', it's no fun. The average woman can handle a man that is 5 1/2 to 6 inches in length. I am now married for the second time and both my wives have stopped having sex with me because I am too large. My first wife started having affairs looking for a smaller penis. My second wife won't have sex with me because it hurts, even if I use half. I always have been told by women in past relationships that I was the largest they ever had. It is a sexual curse to be large and I wouldn't mind losing 3 inches in length. It would put me close to a size that my wife could handle and I would be more at the average size.
A penis is a very delicate and intricate construction of nerves, arteries, veins, muscles, erectile tissue, etc. Size reduction surgery would be nothing like removing fat. Even a slight penis injury can result in loss of function or a curvature. I cannot imagine a surgery that would not have a disastrous outcome. He should be very careful about surgery, even if there is some surgeon willing to do it (there are a lot of quacks).
Women don't like a penis that is 13" long, it is way too long for them. I don't know if there is such a thing as surgery. Your friend should talk to a Dr.
I think the guy should count his blessings! Most guys would give anything to have that size, even though as you say, he has trouble keeping a relationship. if any woman is serious with him there are lots of alternatives that they could explore; anal, manual, etc. But I'd think if you ask me, he likes the variety he inevitably gets and would never seriously consider reduction surgery, because as John said above, permanant loss of function would most likely occur!
I don't think there is any such operation. If there is, I'd be very concerned about permanent loss of erectile function.