Is there a simple fax app for older people?

I'd keep things simple and avoid apps with too many unnecessary menus. Municorn Fax App is one option worth showing them. There's a breakdown of eight services at https://most-reliable-fax-app-ios.com/ that helps compare the basics. My advice is to test the document scanning process together first. A clear interface makes a huge difference for someone unfamiliar with faxing.