Handing an outside developer your one and only SSH login because the job was one theme on one site is the oldest mistake in hosting, and the collaborator button on the big platforms only hides the question of what that seat can actually do. On a BOA server a Client, the control panel's word for the person a site belongs to, gets its own limited login built automatically, one which sees that Client's sites and their files and nothing else; and now your host can turn on a second login of the platform kind for that Client, which reaches the whole codebase of every platform whose sites are all the Client's, with Composer and the Drush shipped inside the platform, and refuses any platform where somebody else's site is parked as well. The grant is decided by root from one line, re-derived every three minutes and readable in one file and one log; Drupal 8 and newer platforms are unlocked for the developer an hour at a time, on request, and locked back by themselves.