If tenants balk or complain, I remind them that when the HVAC goes out and a new compressor costs $700, I pay $650 and they pay $50.  

If I was managing the properties myself and had the time required, it would certainly be somewhat cost effective (ignoring the cost of time), for me to replace stove coils, fix a compressor on a fridge, etc.  

You can always spot them: there have a metal back with no exterior wire grid or tubing and some cardboard peeking out the bottom, where the compressor is.

I Already had to replace an entire AC unit because the seller wired the unit wrong and ran the compressor to the ground.

I have carried my table saw, miter saw, air compressor, and everything else in the back seat of my truck.

It was using the old refrigerant freon... either replace the whole thing and upgrade to puron or buy a used compressor and pay $600-800 for 25 lbs of freon (with luck) for a temporary patch until it needed more refrigerant.

You really shouldn't run the outside compressor in AC mode (assuming you have a heat pump) when the outside temperature is below 55 - and a lot of companies will tell you not below 60. 

(Likely compressor) and maybe she should just buy a new one today instead.

There is one issue the HVAC needs a new compressor.

If no one is occupying the unit and you are in the cold winter months... unplug all appliances including the fridge (they love nesting near the warm compressor motor, and the dishwasher also provides a warm humid environment), open all cabinets and appliance doors.