What will a caterer do if you have a guest(s) with special dietary needs?!
Answers: such as diabetic, allergies, or needs kosher meal?
Any good caterer will make special plates up for those reasons, and also for your vegetarian friends. Sometimes kosher is a problem, because some people just don't get it.
We (our schul) had a passover dinner, once, and the caterer put out Chocolate Cakes for dessert! (the mousse had been backordered or something, and this was a last-minute thing, but they put out flour cake, in the middle of Passover!)
but I've worked for caterers as a waitress, and they just make sure the staff knows how many special meals they are, and where they are being kept warm.
They will either change the menu to suit all needs, or have a special meal preparred for the dietary needs of the guest that requires it.
If you let your caterer know ahead of time generally they can prepare a meal with this person's needs in mind.