The issue to me is professionalism, not weed or booze or whatever. Anything that diminishes the music is a problem. If he can't remember chords when he's high, and he gets high at rehearsal, he's a hack. I'd excuse myself from that group whether or not he got high before forgetting his parts. I'd rather play with stoners than drunks any day, but mostly I'd rather play with pros than hacks.