The Bethanechol prescription came in after the other two, and he wasn't in bad shape until he started getting it. There were some communication fuck-ups, or he wouldn't have been given that last dose of it--he'd already collapsed once.
Things could have been handled much better at this end, but I don't think he should have been prescribed those drugs together.
The vet didn't say anything about things to look out for or possible side effects.
I'm much more interested in making sure this (or similar ill-though-out drug interactions) doesn't happen to other cats than in punishing the vet. I'm going to call on Monday.