For me this is tricky. I do feel it is discrimination, after all they don't do "Men's Night" and what not. And in this day and age of "equal treatment" why does one group of individuals based solely on sex get a discount, but not another? I also find it interesting that so many people who claim to want "equal treatment" either let things like this have a pass, or flat out fight against the removal of something that gives them an upper hand (like affirmative action).
As for if the guy should be suing, I don't really think so. But I do agree such "promotions" like this (or a regularly occuring "Ladies Night" at a bar, etc) is discrimination. But then again, places like Hooters discriminate as to who can be a waitress there, so not all discrimination is wrong/illegal even when it is based off of race or sex.