In most areas where people deal with lions on a regular basis, the dog would be considered a hero. Please research this story some more before you write it, because it sounds like you haven't looked into the laws or actual attitudes about lions in rural areas where they exist.
I happen to live in an area where lions are just part of life. Most aren't a threat and I hope nobody takes this post to mean that I hate lions -- I'm actually glad that the current lion in my area is a "good" lion that seems to leave livestock alone. The ones that are not what I think of as "good" lions are generally dealt with summarily by whoever manages to get the lion in his gun sights first.
Mountain lions are NOT ENDANGERED. If anything, they're overpopulated in some areas. In my area, finding a deer or javelina is almost impossible -- I think the lion's surviving on bunnies and stray dogs.
In fact, they're legal to hunt. I generally buy a mountain lion tag with my hunting license -- a $10 stamp -- that lets me legally shoot 1 mountain lion per year. The reason I do this is that I have livestock, and I have guard dogs for the livestock. I've never had to shoot a lion, but the $10 stamp is cheap insurance to avoid trouble with the authorities.
By law, I can shoot -- without a tag or a hunting license -- any lion that attacks my livestock. But I have to turn over the carcasse if I do so. With a tag, I can then keep the hide and the skull and whatever else -- the hides tan nicely.
By law, I CAN'T shoot a lion if it's attacking my DOG if I don't have a stamp. And since my dogs are bonafide ranch dogs who can and will light into whatever predator comes in the yard (and this is encouraged -- they're ranch dogs and they have a job to do) it's more likely that I'll find myself dealing with a lion/dog fight vs. a lion in with my goats. My dogs HAVE gotten inot it with smaller predators like bobcats and coyotes and feral dogs.
Because of the legalities, I simply buy a lion stamp. That way if my dogs are fighting with a lion, I can shoot the lion. (And in fact, if you have a stamp, you can even use dogs to track a lion and run it to ground. This generally how problem lions are dealt with, unless someone gets a lucky shot off -- like the guy two miles up the road from me who shot a lion that was crouched in the bed of his pickup, stalking his horse.)
What I wouldn't expect would be for my dogs to KILL a lion. I'd be lucky if they survived the fight -- I rather hope they never have to tackle anything nastier than a coyote! Though most lions, if confronted with a pair of angry dogs, will generally run (or go up a tree) rather than fight. THe lion's life depends on not being injured -- if they get hurt, they can't hunt as well. So they tend to avoid fights where they might get hurt. -- Though the current lion by me has an appetite for large dogs and eats a few every year, judging by the kills (called "scrapes") that I've found.
In some states, the law differs about people shooting lions -- but if a couple of dogs got into a fight with a lion, the general reaction, even in places like California where lions are protected by law (much to the disgust of ranchers, from what I understand), would be, "Good doggy! Good doggy! What a hero!" -- not to punish the dog. Oh, the more fanatic tree huggers might throw a fit or two, but I've never heard of a dog being put down for fighting with a lion. Generally, the dog gets kudos from all the neighbors who have livestock and the owner gets bragging rights.
Leva