Mistake in MGMAT advert?

[sudhansu9dm]

Not quite ready to be an instructor yourself? If you refer a friend to us and we hire them, we'll give you $1000.

Should it not be replaced by him or her?

[RonPurewal]

Not quite ready to be an instructor yourself? If you refer a friend to us and we hire them, we'll give you $1000.

Should it not be replaced by him or her?

you are, of course, correct.

the main priority here, though, is not correct grammar but effective advertising; to be effective, advertising language has to resemble colloquial vernacular, not scholarly written language. (ads are supposed to talk to you; at least, research has shown that the most effective ads generally do just that.)
i.e., it would be extremely awkward to write "him or her" in that case. (native speakers of english will universally use plural pronouns in this sort of instance, even though such use is still considered technically incorrect in the written language; no native speaker would ever say "he or she" or "him or her" out loud, excepting deliberate attempts to sound overly formal and/or reading aloud from written materials.)