That surprises me. I would have though there were dojos out there that were a bit more pragmatic. At very least I would expect them to allow her to wear a white skirt over the top of the pajama uniforms.

Obviously it would rule out several schools of martial arts. Karate and Taekwondo would be out for example - too much reliance on kicking. I don't see why she couldn't learn judo, aikido or krav maga with that handicap though.

Then there are options available to her to learn to defending herself while wearing skirts despite not wearing them in practice. There is skirt choice - shorter skirts, fragile skirts and skirts that are easily removable all provide little penalty in combat. Practice in removing, cutting/tearing or moving the skirt to an unconstraining position would also serve her. But even ruling out those options she could simply have training in one of the practical martial arts and just not use any kicks higher than the assailant's kneecaps. Kicks are overrated anyway.