Love your thread title.
Well, I'm sober, and I spend money of my own when I travel for work. Heck, a couple weeks ago I was in Florida and I realized I'd forgotten to pack one of my (approaching 100) t-shirts to sleep in. I wound up spending $20 for a butt-ugly T-shirt from the casino/bar/restaurant where we had dinner, just so I'd have something to sleep in.
So yeah. Stuff comes up. I don't know how long he's going for, or how tight your budget is, but for me, a week-long trip would warrant taking at least $50 for whatever. I use a credit card for EVERYTHING (which I pay off in full each month), so I don't count dollars for the most part (and I can afford not to).
Figure out how much would be reasonable (forget the fact that he drinks) and give him that.