1st...Look up care credit and apply to see if you (well, your mom) may be eligible. It is a credit card that can only be used for medical expenses and they do approve people with lower credit scores than many other sources of credit.
2nd...Call vets and ask. Do you have a regular vet that your dog sees for yearly shots and when they are sick? (You should). I have an amazing relationship with my vet and know that if I were in bind, they would work some sort of payment plan out. They've done it before. But, I have 3 dogs that go in regularly and I have been a steady customer for over 11 years. Now, even if you aren't in that kind of situation (and b/c you have posted this on Y!A, I doubt you are), that doesn't mean there isn't a vet that won't work with you.
3rd...Vet bills are not overpriced. Most mammals have the same basic anatomy that humans have. The cost of a surgery/procedure on an animal is 1/10 or even 1/100th of what it would be on a human. When my oldest tore her ACL and had to have surgery, it cost $1,000. Yes, it's a lot of money, but do you know what an ACL repair would cost on a human?? Yes, some of the meds are over priced but that's the drug company's fault, not the vets.
4th...Yes, a vet can refuse to treat an animal for any reason, including in ability to pay. There are no laws protecting pet health care like there are protecting human.
I'm not judging you. I know the economy is rough. I'm disabled now, and we went from 2 incomes to 1 and we've had to sacrifice. But, my babies ALWAYS get what they need.
If you can't get care credit, can't find a vet to work with you, you can also make a chip in account. Post in on facebook, or any other social networking sites and let people know you need help and why.
I am the momma to 3 fur kids.