I have a need to allow a person to build on one of my plots (near Spawn).

I tried that by first making him a friend: /res friend add {thePerson}
He was online and I see hem as a friend by /res

Then I tried both /plot set perm friend on and after it failed /town set perm friend on

I see that after at least first command, /plot, the permission line has f--- for all types of activities.

He still can not do anything on the plot. What am I doing wrong?

I hope I made an obvious mistake, easily visible from my scarce explanations.
If not, I will double-check and add screenshots, don't waste much time please.