Here is a common problem in bands. You start a band with your good friend. You're not concerned with the fact that your friend doesn't play well. You're just glad to be making music. Over time, it becomes apparent that your friend is not as committed as you are. Perhaps you study and he or she doesn't. Perhaps you practice but he or she doesn't.
The progress of the band suffers and you suffer! The chain is as strong as its weakest link. But you don't know how to handle this problem. After all, it's your friend, maybe your best friend.
This is your first test of developing an attitude of professionalism - or at least seriousness.
If your "friend" won't put the effort into improving, you should replace him or her in the band. Don't let some misguided sense of loyalty stop you from doing what you should do. It's not fair to you to be held back by such an individual.
If this individual is truly a friend, go to the movies with him or her. Hang out, eat meals together, go to a ball game.
Be a friend but don't let it interfere with your future!