Should I be concerned if my dentist is sniffing nitrous oxide during his office hours? Is it safe to have him work on me?

What do you think. If you have witnessed this, then you need to really think about the question. Of course you should be concerned and you should run, not walk, out of there.
Yes. This is unsafe and unprofessional. Find another dentist.
Yes. But you should be concerned if he is sniffing it any time, not just during office hours.