Sometimes the urge to dive overrules reason. I wanted to dive so bad this past weekend that even though my back hurt so bad I was unable to lift my regulator I made my wife put the tanks in the car.
Honestly, two things stopped me. One I considered my buddy and if I could physically help him if I needed to, i.e. would the pain stop me. Answer yes.
Second, I had to have my wife help me out of bed and she even bathed me. The first really stopped me, but the second would have also been good in case reason went out the window.
So yes, I can understand diving with pain. I would have but reason prevailed. IMO after having thought it through, if there is a possibility that your limitations might cause problems on a dive then consider the dive scrubbed.
I think he did the right thing in visiting the Dentist, they have ways to do a temporary fix so he can dive and then when he returns they'll fix it for good, or at least for a while. Sometimes the tooth just isn't meant for this world. Just imagine what could have happened had there been a reverse block in the tooth, the pain would be out of this world.