We have seen it recommended that walls & ceilings be washed with special soap and/or vinegar and lemon water to kill the odor.

One thing we did discover is (this property doesn’t have a basement it’s on a slab), because of the pet urine soaking into the concrete after cleaning, bleaching… we sealed it with Kilz oil based sealer to lock in/out the odors and ran my Ozone machine for a month to help with the smell.The Bear Paint distributor for Home Depot said we should have used water based Kilz and not oil based.

The corrosive nature of soot is driven by the materials that burned, as is the odor.  

@Jazmine S. the ozone is definitely one way, but the painters can put some products in the paint to get rid of the odor.

After de-catting and de-dogging several units from the odor, fleas, hair and damage caused from urine, feces, scratching, digging.

We used respirators and rubber gloves.. the more we removed the stronger the odor became in some areas.  

They could, of course, turn on the bathroom fan (if present) if the odor is confined to the bathroom when they are in there.And it's not going to be resolved. 

As Theodore said the local clubs could be good.  

Also, I wouldn't be worried about a fire, I'd be worried about the odor lingering after they move out and the cost to correct.

I guess one option is to do my best to get rid of the odor but if it still lingers after all the work is complete I could just re-rent it to pet owners.