Will AT&T sue over there mistake?

In 2014 my friend traded in his IPhone 5 for an iPhone 6 and was suppose to get a $400 credit. They made a mistake and he received a $4,000 credit. He didn't say anything because he assumed that they would catch it. An entire year (2015) goes by and he used the credit to pay his bill and to and to buy other things. In January of 2016 he received a check in the mail for $2850, and letter stating that his balance exceeded their limit. Now in 2017 they caught the mistake and are demanding for a repayment. My friend doesn't have the money he is in his 3rd year of college.

Hi so they may try to get it via a court. However the judge may very well tell the company it was there mistake.

Probably not. A couple thousand is nothing to at&t. However, they can refuse to provide him service until he repays