"....wearing skirts and dresses exclusively, shunning pants, shorts, and other such bifurcated clothing....."
In a broader sense, a "skirtarian" is anyone who does this for any reason whatsoever, just as a "vegetarian" may have any of a vast number of possible reasons for not eating meat. Therefore includes a female "skirtist." (see skirtism)
Has she fallen in with some weird religious cult?
No, she just doesn't like wearing pants. Ceremonially burned the last pair she owned!!!