or maybe "a mistake" because he sleep with him while being sober while the other one is completely drunk
(⌒▽⌒)
Ok he was not sober, it literally says that he had been drinking with some people from his work. And there’s a pretty large possibility that once they got ho,e they drank even more. Even though one may have more drunk than the other, both of then had been drinking that night.
There's drunk and there's drunk to the point of passing out. In that case, the less drunk person is counted as sober.
Maybe the "mistake" he means is "sex before get-to-know each other phrase / dating" that why he want to "make it right"