Preferably longer form, but I'll take anything.
https://storiesonline.net/a/Lubrican has a few.
Not exactly what you're looking for (mom's friend): https://storiesonline.net/s/68146/how-i-bagged-a-cougar (PREMIUM)
friend's mom: https://storiesonline.net/s/74816/how-i-met-your-milf (PREMIUM)
sister's friend:https://storiesonline.net/s/44929/knocking-up-sisters-best-friend
Lonny
I listed 3 stories. I'm sorry that 2 are premium and only 1 is actually what you asked for but I thought the others might be good for you.
Lonny
Not friend's Mom but Mom's friend Track and Field by Michael Erickston, although I believe he is 18.
Good Hunting, limab
"His Friend's Mother"
14-yo-old boy and his best friend's mother.
https://storiesonline.net/s/57809/his-friends-mother
It may not be what you're looking for because it's from the mother's POV. It also has some dark aspects to the story with rape.
Ka Hmnd's "Moms Club" ist the story of five 15yo friends sharing their Moms. Problem is it was on XNXX, but all his stories there got removed.
HM.