Not to be insensitive to the concern, but playing devil's advocate -- how much of a problem is this really? I'm talking from a risk assessment standpoint, looking at probability & severity of the risk.
- How long is this stretch of road with no signal?
- How heavily trafficked is it? (if lots of frequent cars on the road, it's easy to flag someone down to help)
- How reliable is the vehicle she uses?
- What is the likelihood of having a problem specifically within this service dead zone?
- How much of a problem would it be to find another means of seeking help? (many homes/businesses that can be walked to if required, or none)
Personally, I would hate to have a totally separate phone & cell service & their associated costs just on the of chance of a low probability, easily overcome problem were to unexpectedly occur. If it means walking 1-2 miles to a nearby gas station or zone with cell coverage, I would totally risk that. Of course, if this is a 25mi deadline with nothing but wilderness around & you drive an old, unreliable car with constant deer/animal "interactions" ... Yeah, maintaining a constant communications avenue might be warranted.
As a different idea to consider... if you really are in a "wide open & empty" type environment, you might check with your local sheriff/state police to see if they have an emergency radio frequency that you could use to call for help if required. Then you could make a 1-time purchase of a long-range (or satellite) radio transceiver for the vehicle, and never have to worry about paying for a service plan that expires or has recurring charges. Buying a simple ELT device could also be an option (emergency locator transmitter, 1-button nationwide call for help, monitored by rescue agencies across the country).