Pokémon/Incorrect vs. Unofficial
Every franchise has official terminology for the people, places, and things that appear in the movies, games, and books, and every franchise has official rules and official pronunciations of words and names invented for the series. If you are a longtime Pokémon fan, you have undoubtedly witnessed fellow Pokémon fans correct others and criticize others for how they pronounce Pokémon names. Pokémon fans frequently argue about pronunciations, just like there are debates in real life about pronunciations of English dictionary words. Purists get irritated when you pronounce a name differently from what is given in official Pokémon handbooks that have pronunciation guides. Talking casually about Pokémon with your friends is a different world from how Pokémon are presented in movies and handbooks and other official Nintendo products.
Official rules edit
Aside from pronunciations, there are two major grammar rules that Pokémon purists insist on, and you risk irritating them if you go against these rules. These rules are official to the Pokémon franchise, so violate these rules only if you are around people who are not nitpicky about the way you talk about Pokémon, or if you are around people who won't go out of their way to needlessly criticize every insignificant minute detail in your remarks about Pokémon.
For the record, deviations from the official information are not "incorrect" or "wrong." They are "unofficial", though if they are widely accepted enough, they can still be considered valid. For example, the official pronunciation of "Ferrothorn" is to stress the second syllable (Fer-AH-thorn), so that the middle "ro" sounds like the word "raw", but most people pronounce it FER-o-thorn, with the stress on the first syllable so that the "fer" part sounds like "fair." This pronunciation, while different from the official version, is used often enough by enough people for it to be a valid alternative pronunciation of "Ferrothorn." On April 24, 2022, when False Swipe Gaming did his video on "How Great Was Ferrothorn Actually?", he opened the video with the clip from the anime showing the official pronunciation of "Ferrothorn", then willingly refusing to pronounce it that way, instead opting for how most fans pronounce it. And if you participate in the competitive Pokémon subcommunity, you will likely have to get used to saying this name, as Ferrothorn is one of the select few elite Pokémon that have been in the OU tier (Smogon's standard tier) for all four generations that it has existed.
The first rule that Pokémon purists are obsessed with is all Pokémon names are to be treated as invariant nouns, meaning they do not change when plural. For example, you may casually say, when talking to your friends, "I caught three Jigglypuffs today", but according to the official rule about plurals, the official plural of "Jigglypuff" is "Jigglypuff." When talking casually, either is fine. "Jigglypuffs" is not incorrect or wrong; it is simply an "unofficial" plural of "Jigglypuff."
The second rule that Pokémon purists insist on being followed is that all individual Pokémon should be referred to as "it" rather than "he" or "she." Again, follow this rule only around people who are not purists, traditionalists, or people who think "mailman" and "freshman" are bad words. Other than that, it is perfectly fine to refer to your sweet Level 100 Blissey as a "she" as in: "My Blissey knows Soft-Boiled and Toxic, and she is amazing in battle."
When to correct someone (or not correct someone) edit
To correct or not to correct your friend, that is the question. The answer may surprise you, so read on.
For the rules stated above, there is usually no need to "correct" your friend for being off from the official way. Even if you know the official pronunciation of Ferrothorn, the widespread pronunciation among fans is not offensive or rooted on anything offensive, so criticizing your friend's pronunciation of "Ferrothorn" is 100% optional. You don't have to correct someone who says, "I found four Zigzagoons on Route 101 today." And unless you are in a situation that makes gendered words unsuitable, you should allow your friend to refer to his or her Hitmonlee with masculine pronouns ("he", "him", "his").
If your friend makes a factual error, such as claiming that Ferrothorn is faster than Blaziken, then you should question your friend's sanity and knowledge about these two Pokémon and their stats. Ferrothorn is absolutely not faster than Blaziken. Not even close. Off by a factor of four, as Ferrothorn has base 20 Speed (and is arguably supposed to be slow, to maximize "Gyro Ball" power), while Blaziken has base 80 Speed and often runs its hidden ability "Speed Boost."
If your friend struggles in a game by not knowing where to find the Bicycle, or even worse, trying to find it in the wrong place, you should give your friend advice on how to obtain the Bicycle. Otherwise, your friend will continue to struggle, possibly fruitlessly, and not be able to continue the story.
If your friend makes a serious mispronunciation of a Pokémon name that no rational, educated person would make, such as pronouncing "Dragonite" as "Bragonite", then you should correct your friend and remind them about English pronunciation rules.