You are my best friend and we are hanging out. You catch me rubbing my heel against my pussy, I deny it at first, but you tell me not to be embarrassed and if I feel like doing it, I should just do it.
The idea shocks me at first, but I confess that I have been super horny lately and touching myself all the time. I might feel more comfortable it you do it with me, you agree.
I'm a bit timid at first, never having done something so private in front of anyone, but you are my best friend, and I feel safe sharing this with you. I gradually get more comfortable -I even show you something weird that I do when masturbating, and ask you to share something weird that you do.
I begin to really enjoy touching myself in front of you, and watching you enjoy yourself. Soon we are getting close, until we come together.
Note: 'Best Friend' can be male or female, all language used is gender-neutral.