I wondered this myself, and Googled it a bit back. I could find absolutely no canon on it. Since there is no canonical answer (as far as i can find) I'm going to give an answer based on logic and extrapolation from canon.
As you clearly pointed out, Lockhart did claim that the Waga Waga Werewolf was defeated by an Armenian warlock, and implied that he'd simply taken the credit.
No one wants to read about some ugly old Armenian warlock, even if he
did save a village from werewolves. He'd look dreadful on the front
cover. No dress sense at all. Harry Potter and the Chamber of Secrets,
While it is possible that the Armenian Warlock did use the Homorphus Charm, I doubt it for several reasons.
- Lockart implies that the village was saved from the werewolf, indicating that once he was finished with the werewolf, the threat had been ended permenantly. This means that one of two things must have been true:
- Whatever method was used to end the threat to the village was permanent, and didn't require him to stick around and repeat the spell every full moon
- Alternatively, he would have had to shown some local wizard how to perform the spell every full moon.
Neither of those makes sense, but the second option is even less sensible than the first. Lockhart was a fraud and couldn't have pulled off "teaching" the skill to someone, so I believe that the evidence points to the solution being a one-time, permanent fix.
So if it's a permanent fix, why wouldn't have Lupin known about it and used it himself?
First, Lupin hated his affliction. Were there a way to restore him to human form ahead of time, I'm sure he would have found it. Lockhart stated that the Homorphus Charm was "complex" indicating that it would take a wizard with prodigious skill to accomplish it.
That's no challenge for Lupin. Not only was he very accomplished, but so were two of his three best friends. They figured out how to become animagi to keep him company. It seems highly likely that if the Homorphus Charm were a feasible "cure" it would be a lot better than letting Lupin run around the Shrieking Shack injuring himself.
And if James and Sirius weren't talented enough, Dumbledore knew of his affliction from the time he was a youth. I find it exceedingly difficult to believe that if the Homorphus Charm were a viable solution that Dumbledore wouldn't have been able to use it.
Given the high level of motivation Lupin would have had for trying any possible cure, and the highly skilled wizards he associated with, I believe that the only logical answer is "no". The best alternative that these incredibly bright, talented, and powerful wizards could come up with for the werewolf they loved as a brother and friend was the Wolfsbane potion, which merely rendered him safe. Were there a better option, they'd have found it.
FAR more likely is that the Armenian Warlock simply killed the werewolf, but Lockhart decided it wasn't stylish enough to simply kill the beast. Any wizard could do that. This is Lockhart, and he has fans to please.
I believe he simply discovered that the Armenian Warlock killed the werewolf, took the opportunity to wipe the warlock's memories, and then made up a story worthy of Gilderoy Lockhart, who not only ended the threat, but did so by curing the poor beast, a feat no other wizard could have done.
There is another possibility: That the Armenian Warlock did actually cure the werewolf, and Lockhart took credit for it. If so, then we'd still need to resolve why his cure wouldn't have come into more common use.
If that's the case, then the most likely explanation is that Lockart had no clue how the Warlock had done it, and made something up. It wouldn't be the first time. If you remember, Molly referred to one of Lockhart's books on how to de-gnome the garden, and even Ron, an underage wizard, knew that he knew better how to de-gnome a garden than Lockhart. So Lockhart, when faced with having to explain how the warlock cured the werewolf, just made something up that sounded good.