The late Esther Myers wrote this
about Endometriosis: Endometriosis is a condition in which endometrial cells from the lining of the uterus migrate to different parts of the pelvis and to other organs. The cells continue to respond to monthly hormonal fluctuations, causing bleeding within the body, which can be extremely painful and can cause scar tissue to form. The scar tissue can itself be a source of significant discomfort. Conventional treatments vary from birth control pills to surgery to removing the tissue of the uterus, fallopian tubes, and ovaries.
She goes on to recommend several postures found in Yin Yoga. I have added the Yin name in square brackets: Start with relaxed abdominal breathing and poses like Supta Baddha Konasana [Reclined Butterfly] and Supta Virasana [Saddle] that gently open and stretch the abdomen and pelvis. Use these poses during times of pain, bloating, or discomfort. Then focus on seated poses that open and release the pelvis like Baddha Konasana [Butterfly] or Upavistha Konasana [Straddle]. Salamba Sarvangasana (Shoulderstand) is particularly beneficial to women with endometriosis when they're not menstruating, since it helps balance the endocrine system. Instead of Shoulderstand, I would suggest Snail, if it is available.
You are correct that in Yin Yoga we do not offer strengthening exercises, but again to quote Esther, To strengthen the abdominal muscles, begin by lying on the back and focusing on drawing the navel toward the spine as you exhale. If there is no discomfort, gradually move into poses that require abdominal strength like Chaturanga Dandasana (Four-Limbed Staff Pose) or Paripurna Navasana (Full Boat Pose). While a woman with endometriosis may be able to do an Ashtanga or Power Yoga practice, I do not recommend this practice exclusively. It would be better to counterbalance a vigorous practice with poses that soften, relax, and open the pelvis. Personally, I am not a fan of boat pose, and would suggest core strengthening exercises recommended by Stuart McGill
: Bird Dog, Plank on elbows, and mini-curl ups.
As with any condition, a doctor's input is important, so it is good that you suggested that. Beyond that, she should experiment: test a little, pay attention and see what happens. Start easy and when safe, go further.
Good luck! Let us know how it goes.