Friends aren't paid to be your teacher. They are friends!
Friends aren't paid to teach you guitar. Teachers are. It is this simple fact that allows me to say with the utmost confidence that asking a friend to teach you how to play guitar will not work out the way you'd like it to.
When you pay a teacher, you get access to opinions that are (hopefully) well-formed, a curriculum that has been tested empirically by students before you, and a guide that holds the space for you as you learn and struggle with guitar. When you ask a friend to teach you, commitment is not really in their mindset. Honestly, why should it be? What are they getting out of the bargain?
My feeling is that your friend will be more willing to play music with you than teach you how to play guitar at all. That's frustrating when you need an opinion to rely on. This is why teachers are the better bet for learning guitar from scratch.
Teachers have ways to show you how to play guitar
Teachers have ways to show you how to learn guitar. This way of learning is often called a curriculum. Curriculums are a system of habits, materials, and skills that guitar teachers have to give to their students. It takes years of empirical testing, updating, and improving a curriculum for it to work for a variety of students. Unless a person has a vested interest in creating a curriculum for students in the first place, he or she wouldn't do it. It takes an enormous amount of energy to create one.
Does your friend have a curriculum?
Teachers hold the space for you
There's a lot of truth to the idea that learning is also consciously coming to terms with struggling. Guitar is no different: Learning guitar requires trying something new, failing, but ultimately succeeding at the same task. It's a special space, that learning space. In my opinion, it is one of the most important and awe-inspiring spaces anyone can sit in. Teachers can help to hold the space for you, encourage you when you're down, and spur you forward. They can hold the hope that you have to learn guitar, even when the chips are down.
Holding the space is likely the most important part of a guitar teacher's job. For me, it's about being present enough to help a student through the difficulties currently being experienced. It's about being there, in the lesson, with no distractions, and being ready to help. It's about giving just the most correct suggestion at the most appropriate time.
Can your friend who plays guitar do this for you? Will they do this for you?
Teachers for learning, friends for merriment
I am firmly of the opinion that it is too much to ask friends to teach you guitar. It will stress them out, it will strain the relationship you both have, and it won't get you any closer to learning guitar. Friends are good for passing the time, enjoying each other's company, and (of course) playing music together. They can challenge you to get stronger at guitar in the long run. Guitar teachers, and the more highly-paid ones, are going to be your best bet to learning.
These guys are really great... by L.A. Shively, Public Domain