Generally speaking, Sprint (and its MVNOs) has the weakest coverage, but is often the cheapest. There's virtually no Sprint coverage where I live. I use Mint for my phone, which has the same coverage as TMobile - Mint uses TMobile's network.
Mint's unlimited plan is easily the cheapest at $30:
https://www.mintmobile.com/product/0...t&subid=3large
If TMobile's coverage doesn't work for her, Visible has a $40/month unlimited plan on Verizon's network:
https://www.visible.com/deals/?CMP=M...oDcQ&gclsrc=ds
Be aware that most of these "unlimited" plans throttle after a certain number of gigabytes.