I have never come across this, and if you think about it, you might understand why. A blood transfusion would require a blood donor - which would obviously have to be another dog. I can't imagine how scared a dog might be at having needles stuck in it so it could donate blood: unlike a human it has no idea it's doing a good deed. Therefore, I would imagine many owners would be unlikely to offer their dogs as donors.
I think your best bet is probably to be there for your friend and help him through the passing of his dog.