I work on this all the time. It is one of the things that many people come into yoga looking for.
Obviously the first thing, which I would imagine you have already done as this is an existing client, is to make sure there are no underlying issues that would affect how stretching can be done. There are structural conditions, injuries, and so on, that can affect how one would proceed.
I will give you a few general suggestions that come from my experience, and may possibly help.
Clearly you want to stretch, but you have to consider what I call compensations.
Also you need to think of touching the toes, not as a goal, but as a tool.
This is what I mean. When I lean over in a forward bend part of what brings me forward is a tilt at the hips that stretches along the line of stretch through the calf and the thigh. But if you have someone who is tight there once that has moved as much as it can, if they are thinking… the goal is the hands on the floor…. they will round the back to get there. So instead tell them not to worry about the hands on the floor, but rather think about bringing the navel to the thigh. If you teach them cat to cow first you can let them understand about keeping a natural lumbar curve through the bend, rather than creating a compensation by rounding the back. Put a few blocks under their hands, or have them do a half bend and put the hands on a wall.
The other thing to do is to do leg stretches where the back is immobilized (laying on the floor and lifting the leg for example). Be wary of seated forward bends as they put more stress on the spine. I do them a lot, but I’ve done them for decades, and I tend to do them with a class only late in the class when warmed up, and with modifications where needed.
Cat and cow, and in particular the 6 movements of the spine are great ways to open the spine. I really think it is important to balance the forward and backward movements with the twists and side bends…. assuming that person doesn’t have a medical reason not to do one of those things.
Also, remember to be cautious with the word ‘best’…. people’s bodies and backgrounds differ widely, and what they need day to day varies. It is generally better to work from where they are when they present, and to modify as needed.
I am going to give you links to a couple of my blogs, in case you have interest…. one is on the ‘leg series’, a very easy set of warm up movements, with drawings. The drawings look a bit like Goya drew them, but that is just my computer incapacity. I will also give you one I did on yoga for runners, which covers some of this territory.
Just remember you want hip openers as well as spinal work, as well as the leg portion of forward bends.