i have a great dane who became really lethargic and the vet originally thought there was something wrong with his legs. xray revealed that he did indeed have a bit of bone where he shouldn't but they didn't know if this was a bit that was torn off or whether it had always been there and they didn't think it was too bad. they could fix it but this would mean going to the uk (they don't do this type of surgery on the island), have one leg done, keep him in very restrictive conditions for 6-8 months after to let it heal (no walkies, no stairs etc), then take him back, have the other leg done and let him heal again for 6-8 months.
he was 5 years old at the time and we just could not justify doing that to him given he quite possibly only has 3 years to live. he had no problems walking but he wasn't running and he didn't like to jump on higher things either.
so we just thought... it's better for him to have a lovely life until it gets bad than go through the surgeries and take a year and a half of his life.
however, we later discovered his thyroid wasn't working and since he's started on medication for that, he's back to his normal self, running like a loonie, jumping and everything!
anyway, my point is i guess that i agree that this poor doggie would probably be better off if your friend let her go. however, i do understand the need for him to realise that as hard as this is, it's the right decision. i'm sure the second opinions will help. and while he's doing that.. you say she's in pain.. they can do pain therapy on dogs quite successfully so perhaps they can keep her comfortable while he's coming to terms with this.