Thanks to everyone for the info - I'll be working on this with the new owner, and also her brother who is more familiar with cars. We'll try everything that has been suggested, starting with getting the car home from the shop.
To me, this doesn't sound like an alternator issue, since the battery still had plenty of juice when I went to try to start it. It cranked vigorously, just didn't catch. If the alternator was shot, wouldn't the battery have depleted while driving? That's what has happened to me on other cars when the alt went. Or am I missing something?
I noticed, while feeling around (mostly in the dark, with no flashlight - duh) the levers that moved the choke plate - under the air filter on the passenger side - that I could feel a little wire spring which was moving quite freely, and not seemingly connected to anything. It was just sort of wiggling, not springing. Does any of this sound like the choke could be part of the problem?
Thanks again to everyone - I'll check back with what we find out.