Tenant shall only use a plunger to attempt to fix a slow or stopped drain, and not pour chemical or other drain cleaners into any stopped or slow drains.Tenant shall also be responsible for any plumbing system freeze-ups occasioned by Tenant’s negligence. 

Make sure your move in gift includes a plunger. 

I once had a tenant with a plunger by the kitchen sink.

“Buy a plunger”.

Buy them a plunger and put a bow on it

I include toilet plungers in my properties.

We actually gave each tenant a plunger bought in bulk and for $2.50 an apartment it was well worth the cost.

CPAs probably don't like it when said mutual clients semi-jokingly ask if they can depreciate a light bulb or plunger (I already know the answer is "no"), but it sets the agenda of "depreciate anything you can possibly depreciate, unless it's going to trigger an IRS audit" clearly.Depreciating $10k in a given year, which might save $4k come tax time (each year for a couple years), is the same (if you divide by 12) as reducing your interest rate on a $500k mortgage from 4.5% to 3.375% (comparing monthly P&I payments), or doing something else to increase your cashflow by ~$300/mo.

Hit it with the plunger and, if that doesn't work, call your plumber that you use for your investment properties.

In either event... introduce them to a tool called a plunger and tell them to open the supply lines allow the bowl to fill, shut off the lines once again, and try unclogging the drain line.