I tried this on my daughter old Lenovo N22 Chromebook.
Using crostini I can get pscan to run, but it doesn't recognise the interface.
I think it's to do with this bug
https://bugs.chromium.org/p/chromium/issues/detail?id=956288#c1
Until Google fix this and roll it out in an update I don't think that it will be possible.
My advice to your friend for now is to buy an old used Laptop (Dell D620 or D630 are ideal, but avoid the ones with the Nvidia GPU) and install xubuntu. pscan works really well with Linux on old laptops.
Your other option is to use a Raspberry Pi and remote desktop into it using the Chromebook.
I am also working on pscan on the Pinebook Pro and it would be trivially easy to release that if he wants something new. The Pinebook Pro is very nice.