This article goes into a lot more detail. It looks like it was just two lines of code. But apparently the codebase actually had backdoors already present. His employer, MSLA doesn't come out looking good. After winning a few lotteries Eddie actually reported the security hole and suggested a fix. It was ignored.
Sounds more like he was able to install software on the same computer to use the same RNG as the lottery. In essence not doing anything to the original code, but using known information about it to get similar enough results.
He did alter the code. See this comment for details: