How is Boxer betrayed? How do the animals discover the truth of what has happened, and what is the pig's explanation?

Boxer is betrayed when he is told that he'll be taken to the veterinary clinic in Willingdon. Instead, he is taken to the glue factory and none of the animals can stop it. It is Benjamin who sees the writing on the truck.

That gave the animals pause, and there was a hush. Muriel began to spell out the words. But Benjamin pushed her aside and in the midst of a deadly silence he read:

"'Alfred Simmonds, Horse Slaughterer and Glue Boiler, Willingdon. Dealer in Hides and Bone-Meal. Kennels Supplied.' Do you not understand what that means? They are taking Boxer to the knacker's!"

According to Squealer:

Three days later it was announced that he had died in the hospital at Willingdon, in spite of receiving every attention a horse could have. Squealer came to announce the news to the others. He had, he said, been
present during Boxer's last hours.