Ah ... those tight hamstrings! Can Yin Yoga help? The hamstrings are muscles (three muscles actually) and normally we use yang yoga to work the muscles, but I have found that Yin Yoga can help with the hamstrings too.
The muscles consist of about 70% muscle fibres but also about 30% fascia. It is more correct to refer to muscles as "myofascial" tissue. It is not really possible to separate the muscle cells from the fascial nets that hold them together. Our Yin Yoga practice can help lengthen the fascia of the hamstrings.
Here is an analogy: imagine a pack of 8 hot dogs (since we are yogis, let's imagine them being tofu dogs, not pork). All 8 dogs are wrapped in a plastic wrapper; that plastic wrap is analogous to our fascia and the "meat" of the dogs is the muscle. But when we open the pack, we find that each dog is again wrapped in another wrapper. This is the way we are: we are basically bags within bags within bags, all the way down to the smallest group of muscle cells. And, again the bags are our fascia.
Yin Yoga can help to lengthen the tight bags that bind us and give us tight hamstrings. There are several poses that can help do this: Caterpillar, Straddle, and Half-Butterfly are some of the best. If your student is really tight, he should start with the legs bent, and support the thigh with some blocks so the can relax the muscles. As long as he is feeling a tug at the back of the legs, he is getting the stress needed to work the fascia of the hamstrings. Dangling would be another possible pose for him but, given the fact that he is standing all day bent over, he doesn't need more dangling.
I am concerned about his lower back. You may want to have him do some easy Sphinx poses and work towards Seal to help move his spine in the other direction. Since he is complaining about lower back pain, even the previous forward folding postures may be contraindicated. If so, start with Half Butterfly and ask him to keep his spine neutral to avoid more rounding of the lumbar.
Beyond just working to lengthen his hamstrings, I would suggest you talk to him about strengthening his core. The fact that he has lower back pain is not good. Until you fix that, I would not worry so much about the hamstrings. To strengthen the core in the right way, you may want to get him doing some yang yoga rather than yin yoga, for now at least. A researcher that has spent his whole career working with people with lower back pain is Professor Stuart McGill at the University of Waterloo. Check out his web site for information on the safe way to deal with problem backs:
http://www.backfitpro.com/ - view his article tab, specifically: Selecting Back Exercises
Finally, since your student is standing all day, according to Mr Iyengar, an ideal pose for him may be the Saddle pose!
Cheers
Bernie