It seems like this is a common problem at SP's house, so it doesn't seem like leaving it is will solve anything either. They already do it, so if SP decides to do something about it, maybe mom/dad will see that there's a waste habit problem that needs to be fixed. Personally, I can't stand the idea of paying for all of that and throwing it in the trash (I work hard for my money--I'm sure SP's parents do, too)! That's the same thing as throwing money away!
If it were me, after a couple of days I would go ask mom if she was planning on doing anything with the chicken. If not, I would then ask if she minded if I gave it away or fed the animals with it so it wouldn't go to waste. I might also suggest, in a non-confrontational way, that wasting chickens/other animals/food really bothers me.