Overripe avocados are a real thing. But they’re not that bad; they’re yummy (if you’ve ever had a rotten avocado, you’ll know the taste).
This article defines if you can still eat an oxidized avocado. Avocado is safe to eat, even if the inside has turned brown. When the avocado’s flesh comes into contact with oxygen, it creates melanin, a color pigment. This is not an indication that the avocado has begun to decay and should not be thrown away.