Let's start with intention: what is your intention as the yoga teacher for this person?
What is her intention in coming to a yoga class? Before we can offer modifications, it is really useful to understand the objective. Yoga in general can benefit anyone in so many ways, but since every body is different, how we can achieve these benefits will vary. I have not personally worked with a lot of very overweight students, but from time to time they have shown up in my classes. One thing that I do know that they really appreciate is to not be singled out. Often they will want to just be in the back of the room, not front and center and just ride the class energy. They are not coming to a Yin Yoga class to lose weight, but they may be here to become calmer, more accepting of who they are, to get some energetic stimulation, to just get in touch with their body.
My general philosophy in dealing with very "stiff" people, regardless of why they are so stiff, is to not overly fuss with them. Even though they will not look like everyone else in a pose, they are still getting benefits from simply showing up in a class and sitting on the floor. I may offer them a couple of extra props, but always I ask for first, "What are you feeling?" If they are feeling something in an area close to where I would like the pose to be working, I just leave them alone. If they are not feeling anything, then I will offer suggestions, or props.
Obviously, the very stiff, the obese or the eldery students are not going to get to the same positions in the poses as everyone else and that is okay. Remember the mantra, "If you are feeling it, you are doing it" and it doesn't matter what you look like, or what anyone else looks like. Having said all that, what about the cases where the student is NOT feeling anything: how do we help them then?
Props, walls and other postures!
Props can be used to support the body so that the student can relax in a pose. Sarah Powers was the first person I heard to explain that, if the bones are not feeling supported, then the muscles remain engaged. When the bones are supported, you can relax the muscles. Props may also allow a pose to be possible in the first place: a strap can help a student reach her toes or stay in Caterpillar. A cushion under the chest or armpits can help a student feel something in Sphinx.
The wall can be very useful for taking a load of the hips and legs, quite literally. Check out the
Wall Yin sequences. If you are teaching a student who is plus-sized in a class room setting, suggest that he come to class a bit early and get a spot by the wall. Then, during the class, you can discretely offer him wall asanas that mimic what you are offering to the general class.
Learn the variations for all the poses. Remember, there are not too many poses in Yin Yoga, so it won't take you long to learn the variations. Also, don't be afraid to offer alternatives: for example, if Swan is just never going to work and lying on the back for Eye-of-the-needle pose doesn't work either, try the
Sitting Swan position. We have lots of ways to work the various parts of the body.
Remember also, just being on the floor will be beneficial even if the student can not get into all the poses. Sitting on the floor opens the hips, stretches the legs and strengthens the spine.
Finally, remember the intention: Yoga is not always just about physical benefits. There are energetic, emotional, mental and spiritual benefits to be found. Don't overly focus on weight loss or flexibility unless that is the specific reason that the student came to you.
cheers
Bernie
ps - You may want to read this
article on weight loss and yoga for children to see if it has any ideas that resonate with you.