In order to get a good culture for a RI, the vet usually does a "lung wash". You need to see what is deep within.

From the mucous you will probably see what is tumbling around in the mouth rot.

This is a very tricky situation and you have my sympathy. Also, kudos for doing all you can to try to save this snake.

So, my frustration is not aimed at you at all, but rather the actual owner. It doesn't matter at all whether she is stubborn, old fashioned or whatever. This isn't about her. This is about an animal she owns, and she ought to give it the best chance at recovery. You would think after owning it for so long, she would worry more about the snake then herself.

As is, the chances of a vet helping this snake are far higher then a vet actually doing harm or causing its death. Especially in Florida there are a lot of exotics vets. They don't need to specialize in snakes. They just have to have the know how and there have to be some like that around !!! Perhaps if you let us know the location, someone here knows a good vet not to far away.
Like Chip said, giving Baytril to such a large snake can do more harm. Especially if you don't even know if it will be effective on whichever germs are present.

I would do all I could to help a friend by helping their animal. But I would insist that we do what is needed. I wouldn't allow my hands to be tied behind my back and being the one that has to watch an animal suffer needlessly. Not doing what needs to be done. Especially since you said money isn't a issue at all. If its all about "stubbornness", then I would tell the friend to take the snake back. Sorry, but I wouldn't want to be the one that watches it decline, knowing all the while it could be helped, but won't be.

I had to watch my Blood Python slowly decline and that was WITH constant vet care of a qualified vet. I could never watch this going on while not doing all there could be done.

Again, my criticism is NOT towards you at all!! You are doing what you can, with what you were given. But knowing that this "home remedy" could do more harm then help, I'm not sure I would want to proceed with that.