Many, perhaps most new skaters have this problem, regardless of equipment problems, because of fear. In particular it feels safe to be on your inside edge, when balanced on one foot, because if you start to fall, you can stop the fall with your other foot. On the other hand, balancing the outside edge feels unsafe, because the other foot isn't placed to stop the fall. Hopefully, the coach can guess whether the skater would benefit from changes to equipment.
Rather than just an extra insole layer under the entire insole, try making the insole thicker under the portion of the foot that needs more support - i.e., under the inside edges.
An alternate technique is to offset the mount of the blade sideways towards the inside of the boot. To some extant this is equivalent to the method mentioned above, but if there is too much space inside the boot, or if part of the foot is not in strong contact, you would still have a skate and blade control problem.
If you can, involve a good coach or other suitably qualified person. They can tell a lot by looking at the alignment of ankles, knees and hips.