What's the use-case for moving towards Linux if they need help with it?

Is there a specific application? Is this just for learning/hobby purposes?

Is he using a relatively simple version of Linux, such as Linux Mint so that everything at least looks somewhat familiar (in this case, to a Windows environment)?

Internet banking shouldn't make any difference at all to what OS he's using, or even the browser of choice. It'll mostly depend on how the specific bank has their website laid out which will determine ease of navigation & use.

Show them text editor VIM

Hope he isn’t swapping one set of (known) caprices for another (unknown) set….. :-)