I'd consider this - (I teach/coach in a different sport both in entry level, and advance skills - I see in action to benefits and downsides to amateur mentoring.)
an individual that doesn't teach often can usually only speak from their own experience, their own anecdotes. As good as they are, and as much info as they have, they are only practiced in communicating a concept from their own viewpoint
a "good" instructor, has a lot more experience, and understands that each person 'gets it' in a different fashion. You don't just teach one concept. You need to be able to communicate each single concept with a handful of different communications and techniques - even better, if you observe how the student learns, a 'very good' coach will be able to figure what teaching example and presentation will have the best chance of working right away. This is because what works for one student doesn't mean it'll work for them all. A "good" teacher is able to figure out which method of relaying a concept will resonate with each student.
Work with a good friend and it 'might' work. Some guys are naturals at teaching. But if not, I have no idea if you have the same starting point as that guy, or if you will learn well from his specific anecdotes from the path he learned to get where he is.....Which is really the main basis in what he has to share....
On the plus side - a friend helping out is usually - very motivated, very sincere, and extremely well intentioned. One can learn a lot from observing a good player and good habits can be picked up. Many times, it's easier to take the input from someone you know well. And, at times, communication can be easier from someone you've already developed a sympatico with.
I worry most about a friend teaching scenario - the friend has the correct mechanics, but have no clue why they, or they even understand it completely backwards - , it's pretty impossible to teach it at that point ------- I see this a TON (beware of lessons from "Feel" players)
At the end of the day, the "friend" is likely only going to be able to tell you how to swing his way, and what's worse, he'll be using his feels to do it.
OK - this is a shorter version of what I was trying to relay. But I'd also say, this is also a trait of really poor formal instructors too. A genuinely good teacher is a real find.