ANY type of smoke from incense, candles, cigarettes, or illegal drugs can be harmful to the reptile.

This is because they are extremely prone to respiratory infections because they lack the ability to cough out waste. The animal could become extremely ill from the smoke particles because it cannot cough them out like many people do when they experience that "smokers cough".

It's harmful in that way, but it will also severely mess with the animals senses. Instinct says that smoke is a sign of a fire, and a sign of danger. When the animal smells that smoke it will become stressed and attempt to flee from the area. It can't really do that in a cage.. so it will further be stressed from the overwhelming smoke smell and possibly expect its demise.

So in other words, your friend is being extremely selfish and uncaring towards this animal.