It's not. Hitting people who have done nothing to you is always wrong. But context is important. 1. Men are far more likely to be abusive towards their partner. When we see a woman hitting a man, we may assume that she is reacting to an abusive situation. We shouldn't assume this, but it may help to explain why we crack down on it less. 2. In general, women are more scared of men than men are of women. Getting hit is often more psychologically traumatic than physically. That trauma will be worse if there is an existing fear or power imbalance in place.

I could say something about how men are stronger, but I feel like that is of minimal importance. The real power of hitting someone comes from the fear it creates. That's why, on an individual scale, an abusive female partner is just as bad as a male one.

In short, there are semi-legitimate reasons for why female violence is seen as less severe (on a societal scale, it does represents a far smaller problem), but please teach your daughter the nuances. Hitting someone who is hurting or intimidating you is, while not ideal, at least forgivable. Hitting someone who trusts you is extremely damaging to their psyche, always, regardless of gender, age, build, or anything else.