Edit save files for multiplayer?

I was playing multiplayer with a friend and he chose the wrong profession. I was hosting the game. Now I know how to edit my own profession, but I can't find his professions in my save file, and he says he doesn't have a save file for the character that he plays with on my farm.
Where would I go to edit the professions of someone who joined my farm? Or is it something they have to do themselves?

edit: I found it myself: the save data for joined players is saved in the data for their cabin

 

Where exactly did you find this?