Once someone smokes in the apt, non smokers won't want it.

In most of California, noone allows smoking outdoors on apartment property. she is saying this is the same thing as a cigarette in hand, and a lighter -- and it was right outside her window which would normally not be allowed. 

Does anyone use air alert monitors in the units or anything that is able to detect cigarette smoke? 

I've seen a gun, smelt weed and seen cigarette ash and an unmentioned pet in all but one of my rentals.

It is legal to have a pet and it is legal to smoke cigarettes.

Such as smoking and leaving a cigarette out which burns the house or some unfortunate accident.

Do not accept a smoker-- not even an outside smoker.

There is a lot of damage however.There is a large number of cigarette burns in the carpet and there are many stains on the carpet throughout the unit.

If there isn't a smoker upstairs and something happens, then you could be sued based on previous warning by the tenant.

The building was fully rented and three of the tenants are long term renters and smokers.