It's the simplest rule: reciprocity. While you never want to promise too much, in general, and you should always take more than you give, in the Scholomance, people feel appreciated when their gestures are returned in kind (and they'll always be on the lookout for cases where you didn't return a favor). So it's easy to tell Ennis she'll introduce him to Jaipur during dinner. Manchester is a good enclave, if not among the biggest or best, by reputation.

Hira explains this to the upperclassmen and her fellow freshmen, briefly, before going off to find Ennis.