To put this in perspective, say the other couple's bill was $5000 JA, (hopefully it was much less than that), if they pay the bill at an exchange rate of 100 JA to 1 US, it cost them $50 US. If they pay at the rate the waiter told them Arthur's charges; 95 JA to 1 US, the bill would have been converted to $52.63 US. A difference of $2.63 US (on an example bill of 5000 JA).
Had the other couple exchanged their US $ at a cambio, bank etc. at an exchange rate of 107 JA to 1 US, and paid the example 5000 JA bill in the National currency of Jamaica it would have cost them $46.73 US.
Maybe the waiter may have been trying to say, are you sure you want to pay in US? (because our exchange rate isn't really that favourable to you). In the big picture, a difference of a couple bucks more for lunch, if one chooses not to use the Jamaican National currency in Jamaica, (NOT local currency as many people call it, but National currency), then perhaps they shouldn't be overly concerned about how a beach restaurant sets their exchange rate.
....When in Rome.....