Jason - anything you can do to help here would be great. The group is coming from all over, not just from Dallas, but that's the only fare I've been quoted. It's for travel from Dec 26th to Jan 2nd.
Theoretically, the afternoon of October 31st would be the day to buy their tickets. Airlines will have opened up seats at their lower rates to sell off unsold seats.
I'm not sure exactly how full flights are the week after Christmas in Belize, but I would think most people are coming home, not going there. If so, then you probably can wait until October to buy. But if it's also a busy day to leave Belize, they may be out of luck if they wait to buy.
Another tip if they buy their tickets on Orbitz. Orbitz offers a price guarantee. If someone buys a ticket for the same flights on the same day at a cheaper price, you get the difference refunded. There is a link on your itinerary page on Orbitz to check if the refund applies. It's a good thing to do before you leave. It's admittedly not too likely to happen if you aren't flying from one of the common Belize departure cities, but worth a check regardless.