I think it's easy to transfer the sadness over the loss of the relationship to the *stuff* because it gives people something to focus on.
Obviously, the OP knows her ex wasn't a real prize. That's why he was the ex!
That doesn't change the fact that it's difficult to see your kids jilted - both financially and emotionally - by someone who should have been a great source of support. The truck was a representation of the dad at least extending *some* measure of blessing to his son once he died, and according to the new wife, that measure was revoked.
I do think hambirg has a good point. If in fact the father *didn't* change his mind, it would be a real shame if her son lived the rest of his life believing that his dad backed out on him. It would probably gratifying for him to know that, even if he didn't fight it in order to get the truck.
I've been in those exact shoes except it was my grandmother who died, and an uncle who had her estate changed from being in a will to being in a trust, and having himself in the position of trustee. Things that were supposed to come to me (that had been supposed to go to my dad, who is deceased)... none of them became ours. He gave everything to his own sons. Probably several hundred thousand dollars worth of assets... all to them. We were not mentioned at the funeral nor in the obit. They got a limo for themselves to ride to the cemetary and didn't even have enough chairs set up for us at the graveside. A number of people called us afterwards to express their sympathy and let us know they thought my uncle's family put on the tackiest show they'd ever seen in their lives. Admittedly, it was nice to be validated.
We knew she had had Alzheimers and so we didn't take what happened personally, but if we'd been led to believe she wanted us to have something and then were told she'd changed her mind, it would reflect on the memory of the relationship.
OP, I'm really sorry your kids were put in this position. I do think that in the long run, they'll do well to hold their heads high and decide to make their own fortunes without that old truck.