Hi Gerry,
Django Rienhart. (I probably spelled his name wrong).
A Gypsy Flamenco guitar player who is well known all over the world. When he was a boy, a fire mangled his left hand and he lost the use of ring finger and pinky. He ended up being one of the most sought after Jazz players of his time. Look him up. He could play like a bat outta hell.
If this man has the mental determination, he can find a way to overcome this adversity, and he will get back to playing every instrument he loves to play. Being a music tutor, tells me he has a deep passion for the art.
IF you need another example, think of Def Leppard's drummer, Rick Allen. He lost an arm, but came back and played on their biggest selling album of their history as a band. Maybe it was the fact that he was a one-armed drummer that helped push the band's popularity on that album? I don't really know, I'm just guessing here.
I guess what it comes down to is this: How close are you guys as friend, (the 3 of you)? Is he good to work with, and does he inspire you to do things on the guitar, as well as your drummer? Those things are important to the chemistry within any band. If the chemistry is there, then maybe it's worth giving the guy a chance, especially if he is determined to succeed himself. Being an outsider, I can't give you any clear direction. All I can do offer a suggestion. In the end, it comes down to you guys. How much do you want it to work?