Force feeding them can be stressful but if any of my frogs looked as emaciated as the frog in the photo you provided, I wouldnt hesitate to force feed the frog as soon as possible. Use a small playing card (make sure it doesnt have sharp edges) or a thin id card or credit card. When Ive had to do this I gently insert the corner (not a sharp corner) of the card into the SIDE of the frogs mouth. a very small amount of pressure and the frogs will generally open their mouths, then you just slip the food in as far as possible. It may help to have someone hold the frog still while you open its mouth with one hand and insert the food with another. Considering the difficulty of doing this, one somewhat larger food item, such as strip of catfish or a nightcrawler may make it easier. Im sure it can handle a nightcrawler.
I would also not hesitate to do as suggested and pick up some unflavored pedialyte to mix into some water and let the frog soak in it for a period of time every day. This will not only help keep the frog hydrated but the electrolytes in the pedialyte will give the frog a boost of energy and help stimulate appetite.
I would continue with force feeding every every couple to three days, depending on its age and rate of digestion, as well as the daily pedialyte soaks until the Frog no longer looks emaciated. If the frog continues to look emaciated after a few weeks, consider that it may have a a disease or bacterial infection.
I would also place the frog into a cage designed with a somewhat more spartan setting where the frog and its cage conditions can be more easily monitored until it returns to good health, such as a setup with very shallow (chin deep when at rest) water, with a land section consisting a section of spnge filter medium such as this
Amazon.com: Black Economic Aquarium Filter Sponge for Fish Tank: Pet Supplies and a hide for the frogs security, using a low wattage red incandescent heat lamp for warmth. I prefer overhead heating of this manner as opposed to a undertank heater or having a "warm side and a cool side" because these frogs burrow (or hide under a log) to escape heat. So while the surface of the soil may be rather warm, it will remain cool underneath and the frog will regulate by either burrowing down a bit or coming to the surface (or coming in and out of its hide). A banking bearded dragon will move down from a basking spot and seek out the cool side of a tank or enclosure, as may a basking turtle, but Horned Frogs are not hardwired to walk off to the other side of their cage a bit, away from the heat if they get too warm.
Your current setup looks very elaborate and hard to clean, and bacteria can build up in these conditions very rapidly. With the spartan setup Ive suggested, you simply remove the frog, dump the water, rinse the filter medium and wipe out the tank.
In a more naturalistic setting I prefer Sphagnum moss over coco fiber, because it makes cleaning a bit easier. I also havent had too many plants survive the rather destructive nature of a large adult horned frog.
Thats my two cents. Good luck.