Easiest way is to change the Domain User password to something else and supply them with the username and temporary password, then change it back after they've finished the work.
Note that "Domain User" isn't your main Panel User. If they require a database to be created you can either do that part yourself, or setup a limited Panel User for them that has access to the domain they're working on and the ability to create databases.
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How To Install PHP.INI / ionCube on DreamHost