The SSA has set aside a group of numbers that aren't issued and will never be issued for purposes of advertising, using on TV/movies, ect. I assume the client will have you use those.
However, those numbers are easy to spot as phony as they all start with 9 (no valid SSN starts with 9, though some EINs do). My guess is if you have any employee who's somewhat versed in the security industry, they'll see right through it.