He should've hired a trapper. Down here in Florida we have roof rats / fruit rats. A lot of people have them without even knowing it. They're mainly arboreal and nocturnal. The way to get rid of them is to hire a trapper / exterminator and once you get them out you need to seal up any small entrances they can get in.
This would include squirrels. Put chicken wire over any vent pipes, repair any damaged soffits, seal up the AC chase and any eaves with cement. It beats the cheap do it yourself smoke bomb idea. Small animals like rats, chipmunks and squirrels don't need much space to get in. A roof rat only needs the diameter of less than a quarter to get in.