I have a rental duplex I have owned for 5 years. I don't live there.

This year both sets of renters have mildew.  Both have heavy condensation inside their windows.  One has heavy condensation all over her toilet tank.  I keep telling her its condensation and she needs to run the fan and crack the window.  She says its a leak.  She's a yoga instructor who smokes pot all day, which I guess means she also is a general contractor.
Both sets of renters moved in fall 2020. Previous renters had no issues.

Any recommendations on how to mitigate this?  Getting these renters to change behavior isn't going to work, and state laws here favor the renter over the landlord.

Any engineering solutions to this?  Windows came with the house and look to be single paned.  Kitchen fan in one unit is not vented outside.

I'm selling this place in the next year so not looking to invest a ton of money to fix this if I don't have to.

Get a whole house dehumidifier. Hard plumb it into a drain.

Wire the bathroom light and fan to the same light switch.

Turn the humidifier down.

If you have any fuel burning appliances, check or have them checked for proper combustion & ventilation / make up air and have flues checked. I found that about 90% of Furnace/Boiler/Water Heater installs were done by those who never heard of the Fuel Code. 90% of home inspections miss the same issues. Get a good plug in CO Detector with a numeric display and make renter use it.

Double pane windows?

Broken seal

There really is no fix other than replacing the units.

Some are not all that expensive.

Single pane windows with condensation means it is cold outside and humid inside - the only way to fix that is to replace the window with a dual pane or otherwise insulated unit, or reduce the interior humidity.  With mildew, new windows aren't going to do it, so you need to look at the humidity.  Cold air outside isn't going to bring in enough humidity to cause condensation, because you aren't going to have more water in the air inside than outside unless the source IS inside.  Do your new renters run longer showers, use a humidifier, grow pot, or other things that would raise the humidity?  Your other possibilities are leaks somewhere.

Mike