Old blind as well as deaf dogs can get along quite well, as many people do, no reason to put a dog down for those reasons alone, as long as the rest of their quality of life is good.
It's not even your dog, so you have no say in the matter, people need to learn to mind their own business, and get a productive life of their own, nor can you tell anyone what to say. I've said it whether you like it or not.
Keep in mind one day you will become old, as well as a grandparent, you may or may not become blind as well as deaf, would you want a stranger, such as one of many none family member to be asking personal questions on a public forum as to what is to become of you and your end of life decision? Hopefully not as this Is None of Your business.
It's you decision alone, and only the closest family members to comply with that decision. You're not even a family member, so butt out. The owner is the only one that should and can consent to euthanasia, in writing, after the vet has explained all options to the owner, not you or your boyfriend. Legally the vet should not even examine the dog without written owner consent.
Just as family members as well as physicians aren't allowed to decide what is to become of an elderly relative that maybe dying/suffering, that decision is the individuals alone, which is generally put in writing (DNR) as well as discussed years before the situation arises.
Of course you're going to say the dog can't do that, and you would correct, however, the dog's owner can, not you, not your boyfriend. Responsible pet owners, which seem to be scarce these days, make end of life decisions for themselves, and their pets, should something go awry and they predecease their pets.and if the owners are alive it's their decision what becomes of Their Pets, Not Yours, whether you agree with their decision is immaterial.
Opinions are like butt holes, everyone has one, keep yours to yourself, you're not even a family member, so even more so you have no say in the matter. You maybe only one many that your boyfriend may have, during his lifetime. Plenty of other fish out there. Grow up, act your age, not your shoe size. I repeat, this is none of your concern.
They actually can cope better than humans, as these senses are secondary to their sense of smell.