(This is my first ever guide so please be gentle)
A weeaboo, if Urban Dictionary is correct, is someone who is obsessed with Japan/Japanese Culture/Anime, etc. and attempts to act as if they were Japanese, even though they're far from it. They use Japanese words but usually end up pronouncing them wrong and sounding like total ♥♥♥♥♥♥♥♥. You can find alot of these ♥♥♥♥♥♥♥ clogging up the forums of Gaia Online, hanging out in the international aisle of the supermarket, or crowding the manga section of your local bookstore. This should give you some insight into a weeaboo, imagine comic book nerds only bigger pricks. If you need visual representation I am happy to provide.

Weeaboos go through many different stages to become the horrid beast of the basement we know today, there are pre stages, before they become a weeb and there are main stages.

Pre Stage 3, Anime Watching: This stage is harmless, it's simply the person watching anime. Basically they have an interest in anime but aren't a little ♥♥♥♥♥ about it.

Pre Stage 2, Minor Butthurt Fan: Once your friend reaches this stage he/she will be more defensive of their TV shows, they may also get into arguments about anime shows. Once they reach this stage there is cause for concern.

Pre Stage 1, Butthurt Fan: Basically an advanced version of pre stage 2, except much more annoying.

Main Stage 1, Weeabooism: They'll be watching anime non stop, they'll start trying to speak Japanese and probably start talking about how they'd love to be Japanese and how Japan is superior. While they're deep up a river of ♥♥♥♥, they can still get out.

Main Stage 2, Sad ♥♥♥♥♥♥♥ Weeb: At this stage the victim will start collecting Samurai swords, they'll almost certainly have an Anime avatar and will probably be in a fair few anime groups, probably own anime games and overall be an annoying ♥♥♥♥♥♥♥ ♥♥♥♥ about their fandom.

Main Stage 3, Cosplay Patheticus: They'll be going to conventions dressed up as Naruto, they'll smell like unwashed piles of steamy ♥♥♥♥, if you look in their internet history there will be a ♥♥♥♥♥♥♥ of hentai, they'll own at least one dating simulator, overall, they're ♥♥♥♥♥♥. Also they'll almost certainly have a waifu.

Ultra Stage, Weeaboius Maximus: This is the level where there is no saving them, they'll go to school (or University) dressed as an Anime character, they'll talk about hentai in day to day conversation, they'll treat their life like it's an anime and they'll cram 'Kawai' and 'Desu' into just about every sentance (or some other obnoxious Japanese word). When a person has reached this stage you must either terminate them or never speak to them again.

So, your friend is a weeaboo, what do you do? Well you can always tell them the truth, that they're pathetic lonely losers who need to grow the ♥♥♥♥ up, naturally your weeb will resist and tell you to go ♥♥♥♥ yourself but hammer it in enough times and they could crack and realise how ♥♥♥♥♥♥♥ pathetic they really are. There is however, no real way to cure a weeaboo, simply put, all you can do is tell them to grow the ♥♥♥♥ up. Weeaboos cannot be cured, they can only realise what they are and decide to stop being a ♥♥♥♥♥♥♥♥, that or they just grow out of it. The only other way to save your friend from the weeaboo ways, is total destruction.