if you quarantine then definitely just shift the one discus, it will be fine by itself, it may even help, i had one discus that seemed to prefer isolation! The spitting makes me think internal parasites and that you may need metro to treat. Without seeing any stringy faeces its hard to say for sure but i had something similar with one of my discus around a year ago, never saw any stringy faeces but the fish would go dark, spit food and turn on its side often clamping one pectoral fin. I treated with metro for around a week and it fixed him up. you can use metro in food or just int he water column but getting it can be the problem often. Even if you can't get the metro i'd try isolating first and giving daily water changes, and keep trying food, but vacuum it out if it isn't eaten