I think $10 would have been fine, but I believe in karma, so what's another $4??
Unless I missed it, I didn't see if you mentioned the profession of your friends....I haven't been a tipped employee since being a breakfast waiter and busboy in High School, but if I recall way back when......it was always the Doctors that were the absolute worst tippers and Psychiatrists were the absolute worst of the worst.

A similar thing happened to me at a client dinner, my boss's boss paid the bill at an upscale restaurant and left a 10% tip.....I was mortified because somehow the client saw the tip ammount and the client was the one who picked the restaurant....he was a regular there! I had to discretely go back and throw another $20 onto the table when no one was looking. I ended up having to do a "Chicago Hat" with my expense report that week to get the $20 back......