Hi Cinza ... thanks for posting your question in the forum. It does make it easy to answer!
The best person your friend should ask about how to rehab her Achilles tendon would be her surgeon or doctor. The surgeon will know just how extensive the damage was and how much she can tolerate while healing. The advice from her Rolfer and Anusara teacher is very general and may not be the best advice for her particular situation.
In general, ALL exercise is a form of stress. When we get a good workout for our muscles, we are actually creating microscopic damage to them, which the body repairs and makes stronger. The body knows how to deal with micro-trauma; without physical stress on a regular basis the body atrophies. So, the fact that Yin Yoga (or any yoga) can create micro-trauma is actually good. The problem arises when we do not rest the exercised areas enough to allow them to heal, or we over-stress the areas and go beyond micro-trauma to major traumas.
Your friend's Achilles tendon is healing, so even micro-trauma right now may not be a good idea ... however! It may be that some yin-like stress to this region is exactly what is needed! When we stress the tissues in a yin-like way we are actually stimulating the cells that help rebuild tissue. Cells like fibroblasts, osteoblasts and chondrocytes become more active when they are subjected to stress. There used to be a theory that bed rest was the best medicine for everybody after an injury. Now we know that that is the worst thing we can do. We need to mobilize injured areas to bring the flow of nutrients to the damaged area, and we need some stress to activate the repair mechanisms.
My experience mirrors the Grilleys; Yin Yoga can help tissues heal, but it has to be done wisely, and after checking with the doctor/surgeon. She is not trying to gain added range of motion right now, but rather she wants to thicken and strength the Achilles. Poses like the Overstepping Dragon may help; if she has enough flexibility, she can work up to Squat. I am sure you can guide her into other poses that will gently target this area. Just start slowly - remember the Goldilock's position! (See Newsletter #1)
http://www.yinyoga.com/Newsletter_volume1.php One to two minute holds at the beginning may be enough.
Cheers
Bernie