I'm sorry that this unexpected expense has come into your life. It's understandable that you were startled by the call, since times are hard right now.
I think that you need to be honest with your uncle - mom can't afford to go in on beef. She doesn't have the money. Period.
I don't know where you live, and what the prices are. Around here Wegman's has large chickens for 99 cents a pound, so a large chicken might be more manageable than other things. Mom could also get a smaller chicken, stew it up, make gravy from the broth, put the meat into the gravy, and the size of the bird wouldn't be so readily apparent.
As far as side dishes go, perhaps you could find a special on potatoes in your area, and just cook them and mash them up. By the 29th people will have eaten a lot of rich foods, and some plain potatoes would probably be very welcome.
I like the rice idea, too. You could add a can of beans and some sauteed onion and some spices and have a tasty side dish that is quite hearty. Something that starts out with a bag of dried beans should be relatively inexpensive, too.
Maybe there is a side dish that you could make with flour and a few eggs, like dumplings or spaetzle.