Give the guest choices:

1. take something else if it can be brought out right away.
2. fix the original dish -- if it takes time, explain this. Then, the choice becomes -- take the other guest's food too. Serve them both, together, a few minutes later. Offer 2 new drinks while they wait. Or -- if it's a big party -- offer the guest waiting for fixed dish something else -- anything quick -- for them to nibble on and enjoy while they're waiting, so no one at the table has no food.

So, there's no debating that there is a problem, and it will get fixed somehow, but the guest is almost always happier if they have some control over the process.