You have to edit the domain to be associated with that new user. Then your user2 will be the only one with FTP access for that domain.
A domain can only be associated with one user. So you friend can have full access to the domain, and won’t be able to access the other domains.
Just a side note, make sure you have a full backup of the domain beore you switch users, someitmes the process doesn’t properly moves associated files.