Have them back up against the wall and put their back knee to the wall, with their foot pointing straight up, shin directly against the wall. Then just lunge the front knee forward (like a quad stretch) keeping that back knee glued to the wall and foot vertical. Their legs should look like an N here. They just keep sliding the front leg further from the wall to open up into a split (once flat, the legs will look like a ring-leap.) But.. time and repetition are the most important. There is not a "fast" way to get a split, but this will help with the correct form.