This saddens me so much because these friends would quite happily spend thousands a holiday or something materialistic yet look at a beloved pet as something almost disposable.
Rhoda I am sorry for the shock of discovering your beloved dog's lump is non-benign 
I completely agree with you.
He is your pal - what's the point of flash holidays & possessions, when your pal needs you?
I think people who are capable of viewing dogs as disposable possessions which should be discarded when they become expensive (probably because they prefer to spend the cash on material goods) & who can dismiss your upset, as well as the dog's life, with "just a dog" remarks are the same kind of people who are capable of dismissing people too. There are plenty of them about - who believe some other people have less intrinsic worth because they are "just a woman" or "just a gay guy" or "just a brown person".
I understand that some people don't 'get' animals, & am certainly not saying that anyone who feels this way is also a people-hater who categorises other folk for dismissal. But the lack of empathy - especially from your DH, who should know damn well what the dog means to you, is heartless.
When he says "I don't want to put the dog through it" is he actually just saying "I don't want to spend money on him"?
He's your dog, & it's your decision.
Borrow the money if you need to.
There are no guarantess, & you may - I am sorry - end up heartbroken anyway & in debt over it. But that is YOUR choice, & you have to live with the outcome. You won't want your memory of his last weeks (IF he needs treatment, & IF he doesn't respond to it) to be tainted by self-recriminations.
I absolutely understand your sorrow & frustration OP.
Why wouldn't you want to prolong your dear friend's happy life?
So long as he isn't having to manage non-stop pain, you keep him going. Don't listen to DH. DH won't be grieving in the same way as you.