Could the severe skin condition of my friend be caused by sorcery or a spell cast by her husband's ex-wife, based on a dream I had?

Visions are not a basis for rulings. The woman's illness may be organic, in which case she should turn to Allah and seek treatment from specialized doctors. If medical treatment is ineffective, then perhaps she has been afflicted by the evil eye or sorcery, and her treatment is through legitimate ruqyah using the Qur'an and Sunnah, either by herself or through a raqi, while adhering to the legitimate Islamic manners.