I indirectly contribute to Flutter (via one of their dependencies), but I would still say to make a web app first, so you can always access your app from a desktop.
I think it’s fine for us to disagree on Mobile vs Web. But I think depends on what his goals are, in my opinion these days Desktop isn’t the main use for apps like this. He could always add that kind of support later.
Having it on the phone is nice, but it has to let me type in my notes at ~100wpm as well ;-)
(Of course, to each their own)