How to Respond to a Request From a Friend?

My sister's friend is throwing a barbecue for 40 ppl tomorrow mixed with adults and kids and she asked my sister's husband to be in charge of the grill since her own husband doesn't seem to like to get off his *** and do things. Now my sister and her husband like to entertain and do it often with lots of detail and preparation when they cook for their guests. But for once her husband would like to relax and be able to enjoy the party since he's suppose to be a guest and so the request seems a bit unreasonable. There will be a lot of people and they are suppose to be the host, right? So my question is how would one respond to this... would they just suck it up and just do it since that's what a good friend would do or is there a way to politely turn them down and still go to the party without it being weird?