However, it's super expensive for a 12 unit building and we have the sprinkler system which I believe would get set-off in the high temperature.  

This would give her the ability to change the temperature while she is out of town.Then she could just text you if there is a package on the porch that needs to be put in the house.

Its a concrete tilt up construction and it has a 30 ft. clear ceiling height.What is the best way to reduce the temperature?

This means around the 1st of the year there is a temporary boom of unqualified renters entering the market and to make matters worse the temperature normally drops tremendously which increases the danger of frozen pipes or high utility bills (choose one or the other) insurance agents tend to frown upon vacancies... 

What was the temperature that day?  

If it has centralized heat already, why not set the heat at a constant temperature, say 65 in the winter, and install baseboard heaters in each unit so the tenants can up the heat to their liking, on their own dime.

. - Jan 17 mother of same tenant tells us there is moisture in her sons bedroom following 2 weeks of -20 below temperatures.

So another factor - is an empty rental in winter temperatures. 

Maybe because of the extreme temperatures here?

Conversely, applying a reflective layer below the roof truss space will help keep the conditioned space of the building cooler in the summer, but may have little (or even an adverse) impact on the temperature of the roof deck itself.