John Green never said anything about her dying a year after, and admits he has no idea what happens after the book ends. (see photo on your left, I also linked it to the tweet in question just so you can make sure it's not fake)
The post you saw that states otherwise was most likely a generated fake tweet. You can literally make fake tweets or posts by using specific apps that provides these services.
Another reminder, fact check your information. Don't believe everything you see on the Internet, especially when they do not cite their sources, do your own research, and always make sure the sources are credible, proof and all. Or risk getting bamboozled.
Not gonna lie, I actually almost fell for it when I came here and saw the discussions and the comments. But I searched it up, and found the tweet where the man himself denies ever saying that Hazel Grace dies a year after the book ends. So consider that rumor debunked I guess.