Referring to a diner in your own party as お客様? Suppose I'm speaking to waitstaff at a restaurant explaining that a diner in our party has an allergy. When speaking to someone working at the restaurant, would it be appropriate to refer to such a person as (seeing as the person is not actually my customer), or should they simply be referred to as ?

No, it would be weird to refer to your own party as as it implies it is your customer in that context (so only the waitstaff would use it). Generally, you don't use to your "own side". That said if you need to honor the person (e.g. it is your client), then I would use .