The way that Boxer is betrayed is that the pigs really do not try to get him any help when he is getting old and sick. Boxer has been about the best worker for the revolution that you can imagine. He has worked as hard as he possibly could and has supported Napoleon no matter what.
But instead of helping him, they pretty much let him die. When he dies, his body is sold to the "knackers" to be taken away and used for glue or dog food or something like that.