And if that gets boring, you could always try, "Is it true that your building's haunted?"
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Where I live, business owners get asked that a lot. I mean, ~I~ haven't asked it yet, but that doesn't mean I haven't been tempted. You could also ask how much of their food is locally sourced (if any), who decorated the interior, which music was playing at such and such a time, what will be playing that night, if they ever have live musicians or dancing, if so-and-so is still working there, if they will be working that day/evening/night, if they have a dress code, if they would allow your friend's service animal (you could choose anything---dog, lizard, cat, monkey, ferret, rabbit, et cetera), if they will substitute one item for another, if they have any items not listed on the menu, or if they will prepare a menu item in a slightly different way than it's usually prepared. You could also try asking them if stock is available, if they cater, or if they will be participating in [insert name of local event or celebration].