That looks like one very tough stain to remove-If the candle was scented then they you have to address the oil used for the scent as well.
So if it isnt scented this may go easier-no matter what it wont be "easy"
Going to need patience for this one-
Wear chemical resistant gloves and a respirator for solvents.
First lets see if they can use some solvents to dissolve the wax-they would want to try Xylene and or Toluene.
They will need to mask off the adjoining surrounding area and floor using a plastic drop cloth.
I have done this on a horizontal surface but not on a vertical surface which I think will make it much more difficult.
I would use some white rags and just blot(let the solvent soak into the sandstone) the stain starting from the highest point and down.
Then use a high quality paper towel to soak the solvent and hopefully the stain out of the stone-
Its going to be tedious work-
Then repeat using denatured alcohol and let dry-I doubt the stain will be gone and next I would try a more traditional poultice using paint thinner and diatomaceous earth
mixed together(peanut butter consistency). The trick will be getting it to stick to the wall. Try placing on a piece of saran wrap and then sticking to the wall.Try and do it in sections.
Keep covered over nite and then remove the wrap. It should come off easily-let the mixture totally dry out and see what is left.
May need to be repeated.
However my best advice to you is hire an stp pro.
I think you will be glad you did.