The poor design probably won't kill anyone (it's a surgical device) but it will cause a greater cognitive load than is necessary.
I have more than enough work for clients who can appreciate what I can bring to the table. I chose the clients I want to work with.
The client didn't knowingly treat me poorly and there aren't many people who work with this particular technology. (The last one they hired didn't know the technology so they hired me to fix it.) I really don't want to leave the guy in the lurch but, as an artist (metaphorically speaking), I'm not interested in doing paint-by-numbers work.
I am wary when customers refuse to listen to experience. Generally what I do is just create two UXes and do preference-testing, either internally, at the clients, or both.