احسن الله إليكم Ustadh, is it permissible for a sister to complete a compulsory 1-year medical internship if it requires treating male patients, solely to obtain her degree, while intending to work only with female patients afterward, if Allah wills?

May Allah reward your sister's care for her deen. The scholars set the default clearly: Shaykh Ibn Bāz raḥimahullāh held that female doctors are for women and male doctors for men, except in cases of pressing necessity, and he permitted a woman treating a man when genuine necessity calls for it, with strict conditions: modesty, avoiding everything that stirs fitnah, and never being alone with a patient.