Alternate History is a real (and popular) genre.
Alternate History fiction asks "what if something different happened" and then extrapolates how the world would be different. Generally, the 'this' is a very big deal and obvious to the reader that it is a departure from actual history.
Examples:
* What if Nazis won Worl War II? (The Man in the High Castle, Fatherland)
* What if a man from the future gave the Confederacy a futuristic weapon(The Guns of the South)
* What if there were dragons during the Napoleonic War?(His Majesty's Dragon)
As you can see they can often(but not necessarily) have a fantastic element to explain why fictional history departed from actual history.
Not An Excuse for Mistakes
Alternate history should not be mistaken for historical fiction that contains a mistake. The point of departure should be clear and obvious from the start - probably featured on the cover or back of the book summary. Alternate History should not be used by a person who wants to write a historical novel without doing his\her homework and tell people all errors are because "Alternate History".
To answer your question of acceptability, here's my answer:
1. The point of departure from actual history vs alternate history should be obvious and have significant consequences. (For instance, changing the birthdate or name of a historical figure would not be alternate history, and more interpreted as a mistake)
2. 454
All departures from actual history should be obvious or easily inferable from the departure of actual history.(e.g. If WWII never happens, then it might be acceptable that <PERSON> never became Prime Minister. It is not acceptable to say that if there was no Spanish Inquisition, then Pope <PERSON> would instead have been Pope <PERSON>) If they aren't obvious or easily inferable, then the novel should explain it.
This Particular Case
If the point of the novel you're talking about was something like "what if the French Revolution never happened", then I would say this was an obvious Alternate History. If the book was a Romance novel that took place in Paris in 1815 with a brief visit to "King <PERSON>" who had rules for 30+ years... then I would say that was a historical novel with a bad mistake. If it was something between those two examples, you'll have to use your own judgment.
Hope that helps! 454