Unless the performer has been allotted comp tickets, the full price of the tickets should come out of the performer's fees. Otherwise, the venue is left paying for the performer's generosity.

Furthermore, the person at the ticket table shouldn't have let those people in until the issue had been checked out with the performer to determine if the email was legitimate. Performers work hard to get their bookings, and most wouldn't risk a relationship with an offer like that.

As it stands, because the ticket seller would neither interrupt a sound check nor make the parties wait for someone to check out their claim, the performer is at risk of losing future work. So much for being "polite".