She should download an e-SIM with an app like Airalo. I use this anytime I travel internationally.

You don’t need physical SIM cards anymore. Tons of different plans, find the best one for your needs.

To switch (on an iPhone) after you install one, go to Settings > Cellular and select the e-SIM you want to use. It usually takes a minute or so before you can use it.

A lot of people buy SIMs from the airport or something, but that’s usually expensive. And annoying - especially if you have to fiddle with removing a phone case and finding a pin to pull the slot out, etc.