[i]A [/i]couple of years back I had a similar situation with a guitarist. Said guitarist was a good friend as were the other band members. We gelled as mates and musically.
But his reliability was just awful. Week in week out we would all be set up at rehearsal and waiting for him to hopefully turn up. Sometimes he did, more often he didn't. Sometimes he would phone, sometimes not.
I stuck with it for about a year or so, until I decided enough was enough. I had come to the conclusion that the situation wasn't going to change and that I didnt want to spoil our friendship with a falling out, which there would have been, as I was starting to feel that it was a real insult leaving us all hanging each week.
So, the friendship was preserved and I'm now in a couple of bands where everyone seems to be equally committed.
My old mates are still at it, although not one gig since I left and only a handful of rehearsals in a year or so.