I was raised in at a time (1960's) and in a place (a small rural New Mexican chile-growing, pecan growing, and cattle ranching community) when/where spanking was a pretty ubiquitous punishment for naughty boys and girls. It was commonplace for boys and girls, from the toddler stage until graduation from high school, to receive discipline from parents, other family elders (grandparents, aunts & uncles, and significantly older siblings & cousins), babysitters (often young teenage girls hired to watch over younger children), other adults in the community (parents of friends, and friends of parents), school teachers & deans/principals, coaches (at school and in private sports programs like Little League), life guards at the swimming pool and at the reservoir, Sunday school teachers, church elders and pastors, the local marshall & the local sheriff, and pretty much anybody/everybody else who was in some sort of authority. If, for any reason, an authority figure did not feel comfortable disciplining a naughty boy or girl, he/she would simply give a report of the naughtiness to the parents and a good/proper spanking was administered later.
Schools administered corporal punishments from kindergarten (~5.y.o.) through graduation from high school (~17.y.o.). In the home, it was commonplace for families to require children to accept family discipline until they moved out. Boys usually moved out at a younger age than girls, because almost all boys either went to college or enlisted in the military (where, in many units, corporal punishment -- the strap -- was still an unofficial disciplinary option for young enlisted men, especially during initial basic training).
Young ladies living at home with their parents were considered to still be under their authority. This was still a very patriarchal community, so young adult ladies were ultimately their fathers' responsibilities until they got married and became their husbands' responsibilities. One of the fathers' responsibilities was to continue to administer discipline as needed, so it was not unheard-of for single young ladies, from 18.y.o. to at least mid-20's, to receive a proper spanking from their father... or, in a father's absence, from another elder male (grandfather, uncle, pastor, church elder, etc) with the delegated authority to watch over a young lady until she was taken by a husband.
In fact, because of the old-fashioned patriarchal nature of the community, it was pretty much understood that it was a husband's ultimate responsibility to enforce family discipline. Therefore, most wives were in submissive relationships with their husbands... and, if they misbehaved, they would would be spanked if necessary.
Okay, so now to the question of friends spanking friends. As explained above, corporal punishment was a ubiquitous form of discipline for naughty boys and girls, for young adult men in the military, for young adult single ladies, and even for married women/wives. So, it was actually not unusual for spanking to be implemented as a disciplinary consequence within friendships and social circles. My first experience with this, at the age of about 7.y.o., was when I broke a promise that I had made to my best friend, Susan. She insisted that I deserved a spanking, and that actually made perfect sense to me, because I knew that I was guilty. So, I quite willingly fetched the small oval-shaped paddle that my mother used to spank my sisters and me. I stripped down to my birthday suit and put myself across Susan's lap. She took her responsibility, to teach me a lesson, very seriously! By the time she allowed me to get up off her lap, I had a VERY red and sore bare bottom, which was kept on display when Susan sent me into the corner for 15 minutes of cornertime.
Another example was when I was in junior high school. My closest friends and I formed a club... "The Red Dragon Club". My dad actually built us a clubhouse on our small ranch, down by the river. Official membership in the club was limited only to boys, but girls were invited to be "Red Dragon Little Sisters", and they were considered to be honorary (non-voting) members of the club. If a Red Dragon or a Red Dragon Little Sister broke any club rules, he/she was required to strip naked, go outside the clubhouse (naked) to fetch a river willow switch, and accept a proper thrashing (a predetermined number of stripes across the bare buttocks) in front of the others. It was at this age (puberty) that many boys and girls started to realize the the sexual tension/arousal and embarassment that often accompany a proper spanking. Boys were especially embarrassed when their erect penises were on display, and throbbed against a girl's thighs during a trip over her lap.
Also, starting in junior high school and continuing until today, Domestic Discipline spanking has been a component of all of my romantic relationships with girlfriends (including the GF who eventually became my wife).
From a young age until today there are too many examples of spanking friendships (with me as the spanker, the spankee, or both) for me to remember/count!