Friends aren't paid to teach you guitar
Friends aren't paid to teach you guitar, so why expect them to be your guitar teacher?
Teaching guitar, believe it or not, is a full-contact sport. Guitar teachers must know how to play and how to teach guitar. They get paid because they have developed a method that they use to teach many different students. Even if you paid your friend twice the amount you'd pay the nearest guitar teacher, he still would come up short because a dedicated guitar teacher has experience to teach you what will work and what won't.
Teachers know the pitfalls. Friends, bless their hearts, are usually quite clueless.
Friends are equals, teachers are superiors (for better or worse)
Another huge reason why asking a friend to teach you guitar is a bad idea is because it messes with the established power dynamics in your friendship. Teachers have power because they have experience teaching guitar to many different people. They have a shortcut to guitar. They (should) know the best way to help you, and because they know the way, they are the guides. You are asking them to take charge and guide you, and you're paying them out-of-pocket for the favor.
Friends are equals. Asking them to be your teacher and guide you through the wild world of guitar will change the power dynamic between the two of you. Is this what you want? Do you want to make your friend who plays guitar into a superior? Do you want to see this friend, someone who most likely doesn't have a curriculum and won't be paid for teaching you and won't take responsibility for teaching you week after week and probably can't answer all your questions about the guitar, as the superior? I hope not. Friends are too important for that job.
When it comes to choosing who you are going to learn from, I hope you'll consider choosing a guitar teacher. Friendships can be long-term as long as both of you are getting what you need from the relationship. That will get flipped on it's head once you ask a friend to teach you guitar.
Does this mean that you shouldn't play music with a friend at all? Certainly not.
Ask friends to play music with you, instead
I think it's perfectly okay to ask friends for their opinion on specific issues on guitar every once in awhile. Just don't expect your friend to know everything. That said, most guitar-playing friends are far better as guitar buddies than they are as teachers. Why not just play music with them?
The great part about having a friend who plays guitar is that you'll have someone to challenge you and inspire you to get better at playing guitar. If you are concerned about how long it takes to be good at guitar, than having more guitar buddies to play music with can only help you.