Sorry, brisker. You are wrong here. While beggars can’t be choosey, and mytake did admit to sitting uncomfortably because she didn’t want to be offensive to the volunteer, the party that was out of line here is the driver. She was thoughtless and inconsiderate of the recipient – and that is at least half of any chessed project.
While it might make a lot of sense to add an extra mitzva on your way home from a wedding, a) leave when you say you will b) if you offer a ride to other wedding-goers, remember that the party is over as soon as you pick from a hospital.