I can't say that I think letting him say goodbye was cruel. I think they understand more than you give them credit for. He was there when it happened anyways so its not like I could have shielded him from it. We do intend to get another boxer but through a proper breeder, which can take time. I doubt the dog would think a new dog is the same dog they lost.
No, they understand far more than you give them credit for.
If the Boxer died in front of your Mal (new info), then of course you couldn't have shielded him from it. But there are folks who insist on dragging their dogs up to the dead bodies of their friends when the dogs have not seen their friend die.
My dogs have always known when I have taken a dog away to be euthanized, Sometimes they howl in anguish when I leave with the dog. They know the dog is sick and cannot be saved and will not be coming back. That is how much they understand.
I have also had dogs drop dead from hemangiosarcoma in front of their dog friends. This is much, much harder on the dogs than taking a dog away to be euthanized. Obviously you couldn't help it that your Boxer died in front of your Mal.
Whenever one of my dogs has lost a beloved dog companion, I have moved heaven and earth to get him a new compatible dog buddy as soon as possible. There are lots of good dogs and lots of good breeders, and it has never taken me more than two weeks. It was really inconvenient for me, but that didn't matter. I made it top priority. And I have never blown off a dog's grief as mere "pouting."
And whether you believe or not, an older, perhaps somewhat senile, dog can come to view a newcomer as the dog he lost. I have seen this happen at least half a dozen times.