Nipping issues in the bud early on usually saves time and money in the long run. When your tenant calls to let you know that there’s water coming in through the floor, wall, or ceiling then popping over to try and fix the problem quickly is a really good idea if you don’t want to end up with a bigger bill.
This landlord obviously didn’t get the memo, though. When his tenant told him that there was water coming into the apartment, nothing happened. The water kept coming in for a few weeks before this little funghi popped its head up over the carpet. So much water that you now have mushrooms is not going to be an easy fix.