If the avocado has actually spoiled or has grown moldy then you might get a bit sick such as having an upset stomach, vomiting, diarrhea or nausea.
But other than that eating a spoiled avocado should not do much harm to you other than give you a bit of less severe food poisoning.
Still if you did get food poisoning from the spoiled avocado the sickness should go away within a week or even less but if you still feel sick then it could be a good idea to go to urgent care just to be sure everything is okay.
If you think an avocado or any other fruits or foods are spoiled then it's best to just throw it out and not risk getting sick over it.
But if the avocado was just brown it's usually still very safe to eat and due to a chemical in the avocado reacting to the air just like apples sometimes do.