San Diego official temperature seldom is above 90 degrees.  

We took the highest month of "normal" winter usage we had records on -- people in the house, heat running at a reasonable temperature, washer/dryer running, etc.  

It might be worth talking to a lawyer but not sure if it's small claims or worth it.Also, you might already have all of these things but for future prevention:- smart thermostat (so you receive alerts when the inside temperature dips below whatever preset you have)- Minut (or similar device) would also alert you to the inside (and outside) temperatures; for example, I have 2 devices inside the house and outside on the hot tub deck.

The tenant frequently informs me the interior temperature only reaches 68 degrees.

Yes, this seems to be happening a lot now as a way to advertise without advertising, so I wanted to check the temperature to see if it's me or everyone is bothered by it.

If the temperature goes below freezing point, pipes might burst. 

Where I am the current temperature is about 10 F right now.

If the temperature goes below freezing point, pipes might burst.

The problem ended up being so severe the pest company had to tent the house and raise its temperature to something like 110 degrees for a few days.

You can probably find an online link about how extreme temperature changes are not the ideal way to heat a home and cost more in the long run.I don't know what the local rental rules are where you are, but look them up and see if you can give her notice.