Let me state my standard caveat: Yin Yoga is not restorative yoga.
At least, not the Yin Yoga as described in YinSights and taught by me and others. This does not mean that the principles of yin can not be applied in a therapeutic way: it can. It has. (See the post on this topic:
http://www.yinyoga.com/forums/viewtopic.php?t=466 ) But the way we teach Yin Yoga traditionally assumes that the student is well. If they are injured, they need to look after the injured area before trying to strengthen it.
Okay: that's the disclaimer. But you do ask a legitimate question. And my first suggestion is - if your student is really concerned, she should check with a health care professional. (Hmmm...I guess that this is my second standard caveat.) Unless you are a professional, yoga teachers should not be dispensing medical advise. You don't have the training, nor the liability insurance to cover it
My suggestion for the student with soreness is to first, leave it alone. I know how hard that is. I used to play a lot of tennis and constantly had tennis elbow and shoulder issues. They did go away, eventually, once I stopped playing! Her shoulder is injured. It needs to heal. If she keeps playing through the pain, it will not heal. Her choice of course. Yoga is not a miracle drug: the mental side of yoga is what she needs right now. She needs to practice acceptance and compassion for her body. Let it rest. Once it is healing, she can start to help it heal with some gentle restorative yoga. Slow movements, not to the full range of motion but just enough to mobilize the flow of energy to the area.
For the student who is tight, a very different story. Yoga will help her. But, in the yin style, don't go too far too fast. She didn't get tight overnight and she won't loosen up overnight either. Read this post for some shoulder Yin Yoga ideas:
http://www.yinyoga.com/forums/viewtopic.php?t=341.
I hope this helps!
Cheers
Bernie