I am now a little wiser, but only a little. I have a mail account at web.de, and it offers webdav.
I can transfer files to there with
curl -T localfile --user firstname.lastname@example.org 'https://webdav.smartdrive.web.de/dir/'
curl -Xdoes not work (see man curl).
-Tuses old http PUT and not a new WebDAV command.
The following worked to create a directory dir:
curl -X MKCOL --user email@example.com https://webdav.smartdrive.web.de/dir/
Important was to add "/" at the end. And the following deleted it:
curl -X DELETE --user firstname.lastname@example.org https://webdav.smartdrive.web.de/dir/
If he is used to the command line,There is 'sshfs' as a solution otherwise
scpare enough. He can do things like:
ssh user@remote ls dir
scp localfile user@remote:dir