<< First of all, for the fleas and ticks, do what several others here have recently suggested. I'm also wondering why you haven't done this before, or made your GF do it.
As for heartworm medecine, you HAVE to have the dog tested first, because if the dog already has heart worms, the heartworm medecine will kill him/her when applied. To clarify, giving heartworm medecine to a dog already infested with heartworms will kill that dog.
Re:euthanasia. Your joke profile is uninformative as to where you live. Many shelters in larger areas euthanize dogs with decompression chambers, WHICH IS A VERY PAINFUL WAY TO DIE. So, why do they do it? To minimize the traumatization of the poor animal loving workers who must perform these operations, that's why. It's just a little less personal than holding a dog down while you inject it.
As others pointed out, $200 is ridiculously steep for euthanasia. When my Great Dane (immobile from bone cancer) needed to be put down, my friends and I put him in the back of my (then) Datsun wagon, and we drove him around as close as we could get by car to many of the wooded haunts in which he and I had roamed. Then, I paid an understanding vet $60 to come out to my house (also in the woods), and administer the coup de grace in the sunlight, amidst familiar sights and smells, surrounded by his human friends, after a last meal of steak.
I don't know the full extent of your relationship to and love of dogs, nor do I know the same about you and your GF, but, again, as several have ventured to opine, with the right care and treatment that pup could probably still thrive.
Why not shoot your GF instead? >>
Your response was great up until the last line. Then you got "gonged." I don't know whether that last line was some subliminal attempt to get me to reconsider my "shoot the dog" stance, or whether you are just a real idiot. Doesn't really matter, actually.