We were always taught the following regarding smoking on the flight deck:
1.It's not good for the gyros and other instruments
2.Bad for the night vision
3.Can make one more susceptable to hypoxia
4.Big no no when the O2 is on
With a grand total of maybe 2 gyros in current cockpits, perhaps #1 would not be your best argument.
How about waiting for the next time an engineer has to make a visit to the cockpit, one could privately mention to our butt-puffing hero that the engineer noticed the distinctive odour of tobacco smoke.
