Of course, if this is a real concern for your student, she may want to seek professional advice. So, what follows is some very "unprofessional" advice.
Tingling of any sort is not good during our yoga practice, so you are right to try to find alternatives for your student. Experimenting with feet position may work, and you may want to suggest bending the knees too. This will release the hamstrings and reduce the stress on the lower back. If sciatica is the issue (which it doesn't sound like it is), bending the knees may help. But even if this is not a sciatica issue, bending the knees may reduce stress on other tissues that may be restricting blood flow. Another option: sit up a bit higher on a cushion or two.
Generally in meditation, pins and needle feelings are often observed when the student comes out of the pose. You don't mention whether your student feels these sensations right away, or only after a while or when coming out. This discomfort is usually caused by restricted blood flow; once the blood starts to flow again, then the pins and needles start poking. Immediate feelings of tingling, however, are not normally caused by blood restriction but by nerve impingement. It is valuable to know the probable cause, because the solutions will vary.
One other idea: hold the poses for shorter periods. Experiment to find out how long she can go before the sensations begin, and come out before then. Massage the legs, and then go back into the pose. The total time is the key in Yin, but you don't have to stay in the pose for that length of time all at once. Going in, coming out, going back in ... may work for her.
Good luck!
Bernie