Your student asks a good question! Of course, like always, it depends on the student and her condition, how intense her practice is, her age, etc. But in general I feel a Yin practice can be done every day.
The idea that we need to rest between periods of exercise is a "yang" idea. It is very true that between strenuous muscular exercises, the muscles need a good day or two to recover. And so, we are taught to work one area one day, and a different area the next day; thus allowing the first area time to recover. Again, this is good advice for yang tissues and yang exercises. Yin tissues are different.
Here is one study
that shows that we can (and should?) continuously stress our yin tissues, and rest days are not needed.
Here is one excerpt from the study:
...it can be concluded that the clinician's ideal treatment program for a patient with nonosseous, passive joint limitation should be mild stretching, as much as is practical throughout the 24-hour day, 7 days a week, and to start this program as soon as joint motion is allowed.
Now this study is from a company that sells dynamic casts, ones that continuously provide a very mild stress to the connective tissues. They obviously have a self-interest in this conclusion, but their findings from our Yin Yoga point of view should remain valid. There are several studies that show going to the maximum stress is counter-productive. A moderate stress is best, and then let it linger (marinate!) for a long time. Too little stress is not good but neither is too much. (See my recent post on this topic here: http://www.yinyoga.com/forums/viewtopic.php?t=404
A yang rainstorm is sudden, intense and brief (ie: like a tropical monsoon.) A yin rainstorm is a soft drizzle that can last for days (ie: normal Vancouver rain). Our yang/yin yogas should be similar. I would make an exception if your student's yin practice is too intense. If she goes right to her final edge every time, then she may be tearing the connective tissues, and they will require time to heal before she does another practice. However, if she goes to the mid-point (the optimum point) of her ability, enough to feel a good sensation but not enough to be sensational, then she should be able to this practice every day, and even several times during the day! That is what the study showed.
Having said all the above, I will add one final caveat: if your student feels any tingling during or after a yin practice, she needs to investigate this and perhaps back off. Tingling is a sign that nerves are being compressed and/or damaged. Over time, we do eventually open up to our full, natural range of motions. For some people, before we even get to that ultimate limit, because of the ways our nerves are threaded through the body, we may start to press onto them. That is not good, and if the nerves are affected and start talking to you, time to lighten up.
I hope this helps