It's dangerous in the sense that your dog doesn't understand how to interpret the effects, so may do something irrational or out of character harming either himself or someone else..... But whether or not it's "physically harmful", it is entirely immoral and unethical... It tantamount to blowing smoke in a child's face.. And you have no way of knowing, how the experience could affect his overall psyche, because he has no category for the experience, he could develop long term issues with trust, fear, paranoia... It's just plain wrong. 

 Any "unknown" substances to the animal could be harmful. Smoke, alcohol etc, as the dog's system has not had to deal with this type of ingestion.
 
If smoking/ second hand smoke is harmful to humans, dont you think it is even more harmful to animals? Please, it is irresponsible of the person who did this.
Yes it was once off, but it can have affected his nervous system (which you will only be able to pick up maybe later in its life)
 
Moral of the story, do not do things to an animal that a right minded person would not do to an infant.
 
They were put on this earth to nurture and care for, not poison and abuse.