When you go on holiday, do you have to find an Internet café to print off your boarding pass for the flight home? Or do you select destinations covered by airlines that allow you to get a boarding pass more than 48 hours in advance?
They said they don't use a phone. You can do online check-in on any device with a browser. I'm sure you can point to a single airline that only offers it through a native mobile app but that'd be an extreme exception.
Most airports have kiosks to print boarding passes. It isn't the end of the world.
Also luggage check-in counters will print a boarding pass.
Most hotels would be happy to print your boarding pass for free.
Which airline requires you to print your own boarding pass and won't let you on the plane without a phone? Now I haven't flown in years but last time I did, the normal procedure was to show up at the airport either with your printed ticket or just your reference code and ID, and either use a kiosk or talk to a person to get your boarding pass, and also to check in your bags.