Hardly?
I don't understand. At most, you can say it is justified discrimination.
She will see guys wearing tux or, not required to wear a DRESS for the simple fact they are male instead of female, to the same event.
Just to be clear -- since this is clearly a semantic argument -- discrimination cannot by definition be justified.
If it's justified, then it's simply a (good) law or rule, and not discrimination. Discrimination is specifically the act of treating one group or class differently than another group for no good reason. For instance, there is good reason why men's and women's sports are separated; we recognize real differences between men and women's athletic performance (due specifically to hormones like testosterone), and recognize persistent differences like height and muscle mass. That isn't discrimination because these differences are valid and pertinent to the situation. Similarly, some sports separate people who weigh 100lbs from those who weigh 200lbs, such as wrestling or boxing. Again, this is because we recognize real differences in physical performance between these two groups and work to separate them.
So again, there has to be a good reason. There are lots of bad reasons, and one of those bad reasons is "because that's the way it has always been."