This.
The issue with doing this type of event, is that you end up making your own life a lot harder (and there is the risk of screwing it up by forgetting and using the wrong utensils).
You should offer a meat option and a veggie option. Thats your basic minimum.
Your choice then is either to go halal for all the meat OR to go to non-halal for all the meat.
Then that gives the choice for those who take issue either way.
My considerations here would be the cost difference for halal v non-halal and who your target market is.
You are doing the event to make the most money possible.
You will always sell significantly less veggie burgers, as people are less likely to switch.
So for me it would ultimately come down to what % of your target market will be Muslim. If its a lot, ultimately there will be fewer people who don't want halal. If its only a few Muslims, then your shift more burgers for greater profit if you go no-halal.
Your husband's personal preference shouldn't come into this. This is a numbers game to make money.
If you piss about with halal / non-halal burgers its more equipment, more people to man your stalls and more risk. AND you risk having shedloads of burgers of the wrong type left at the end that you can't shift.
(Yes I have done lots of this type of stuff for charity before - never lose sight of the fact that its about making as much money for as little hassle as possible. You want to keep people happy, but there are limitations to pleasing everyone - as in you will never please everyone the whole time, so accept this and move on).