I agree that people in that situation should be able to work some of the time. Just the same as teenagers, part time workers and legit "extra income" people. This is always the argument against, say, raising the minimum wage, or providing more workplace protections.
Its a lie - you can absolutely fix this situation. No law is gonna be perfect, but having a situation where the "main" job provides actual days off and health insurance is a good thing. If disability is the main job, and you get benefits through that, nothing should stop you from doing part time.
But, you shouldn't allow your friend who already gets a disability check to dictate that everyone being abused by a second class labor system shouldn't have rights too. I would RATHER your friend who is already taken care of lose their gig economy job if people who have to work 3 jobs instead only have to work 2. I'm sorry, thats better overall for society, to me. You can argue the tradeoffs, and maybe have people on disability be able to work 20 irregular hours without losing disability. But you're depriving people of time off to be sick, time to be with their families, and health insurance so your friend can get slightly more self-actualized while he's actively getting a check every month? I'm being somewhat trite here but, in my mind its not even a comparison.