overweight and sun salutations

Last night I had a very large woman in my class. She wasn't there the week before, so I kind of forgot about her when I was preparing my class. So we did quite a few sun salutations last night. She of course couldn't go down & get up, downward dog etc. right. So she knows a bit about yoga, and it's funny, every time I'm talking and explaining something - and I pause to collect the right words - she speaks the words for me. she's the only one in the class that knows what sun salutations are - says she does them out of chatelaine mag. So she can't keep up and she just says to me, I'll go on my own, so she does a bit and then sits down & watches. She is very aware that she's heavy & she even demonstrated how she couldn't get out of downward dog to standing, as she tried to move her belly out of the way. I wasn't prepared for that, I should've been, but hindsight right. So I feel bad, because she could've done the half sun salutation and then went into warrior 2 or another standing posture, while we were on the floor. not? Has anyone had any experience like this? She seemed okay, but it wasn't fair to her & I want her to enjoy her class and take part. She has a bad knee and says her knee already feels stronger. Anyway, any thoughts. Thanks
Roberta

Could she use a chair as a prop? Do a more upright downward dog with her hands on the chair seat. She wouldn't be putting a lot of weight onto her hands but she might get some hamstring stretch and be able to try to lengthen the back and move the shoulder blades out to the sides. She could also use the chair for some of the bending over parts of sun salutaions like uttanasana. Warrior 2 or a high lunge (maybe using chairs as a support, although that would take some moving of the chairs).
I'm not sure but I vote chairs.
Was it just the up and down parts or bending that was hard for her?

This is a challenge I have dealt with before. I always feel like I don't want to single them out becasue they are over weight, but I want them to get something out of it too. I don't know what to do to help them. I just tell everyone to do what they can and take breaks. I move slower if I find that someone can't keep up.
I am looking for some suggestions too!