I am sorry to hear that her ovary and tube were removed (salpingo-oophorectomy). Unfortunately, this can cause reduced ovarian hormone production which can negatively impact not only fertility but other aspects of health including mood. Sometimes these hormonal changes are temporary but they can also be permanent.
Have her periods returned to normal? If not, a hormone panel of estrogen, progesterone, testosterone, and FSH may be helpful. Birth control pills are usually helpful in leveling out hormone levels so she could try that. She is at risk for early menopause without the one ovary so that should be kept in mind as early menopause is associated with some increased health risks. Hormone replacement mitigates some of these risks. If she wants children some day, obviously that is a concern and she may need to work with a fertility specialist.
Best of luck in getting her the help she needs!