A woman wanting men other than her husband to think of her sexually is sinful, because it is enticing to lust. A man thinking sexually of women other than his wife is sinful, because it is allowing for lust. The problem is not with the skirt, but that doesn't mean that skirts are ipso facto OK. It is a meat-sacrificed-to-idols issue: a Christian woman may know that she could wear a skirt without sinning, but if it causes others to sin, then she should not wear it, not because of her own conscience, but because of the viewers'.
BTW, Straw Man on your part.
How can you prove that she is trying to entice men?
Only God can see the heart, and if God tells her that her shorts are long enough, then who are you to tell her (or him) otherwise? You were never be called to be the Spirit that she or any other person walks in so that they will not fulfill the lusts of the flesh.
And no circular reasoning, either: “If she were truly hearing from God she wouldn't wear shorts, because...”
Can't have women wear that which is enticing to men, can we!?
But who decides what is enticing...or isn't? God alone.
To you, seeing a woman's hair isn't enticing, but to many Muslim men it is. Christians taking the same path that Muslims take isn't a good road to trod.
We don't need to go down that road...
God is really smart, He knows how we work, He made His standards for a reason. Moving a fence that you didn’t build, is very dangerous.