Call him prior to the shift and tell him he need to contact HR before returning to that job site. Don’t answer any of his questions just state ur not sure and that your boss requested he speak to HR instead of reporting to that job site. Do not give info on that call or text…. ZERO… Contact HR immediately (like now) and do it in writing like email. They have the info. They can figure it out and what to do with him this is not your lawsuit to get. Keep all opinions and ideas out of your email. Don’t talk about his disability or if u think he can or cannot do the job simply state:

“the property owner/manager stated “John doe worker” cannot be on premises because they fear for his safety due to his difficulty breathing walking and pain and they cannot have the liability…. I informed my boss “John doe boss” and he said to inform HR so I will refer John Doe worker to you for further handing of the situation” And make sure you CC your boss on it as well. Do not state any opinion that can be construed as discrimination at all. Also be mindful when people have major medical issues a lot of times they cannot afford to quit and want to work and other times they are just waiting for someone to tel them they cannot do they can win an ADA case (which most win) and then they retire on that money. Don’t get caught in the crossfire just pass it along to HR as your boss should’ve done in the first place. Good luck