I agree with Chris. It's important to know from the start what you want to get out of the coaching relationship.
As to being coached by a friend, it'd depend on who the friend is
I've been coached by friends and have also coached friends to coach each other where nobody gets paid, and the results really depend on the dynamics between each other.
For example, one of my friends is a really driven nice guy, and we click well and so had fun pushing each other. But I've also had free coaching relationships that didn't work out because the 'buy in' wasn't there, people tend to put more in if they have to pay for it