If not claimed as a dependent on the parent's 2019 return, do you know if the SSDI person would qualify for the $1200 stimulus tax credit next tax season by filing a 2020 return? 

Obviously, the parents would not be able to claim the dependent again in 2020, so they would forego another $500 child tax credit.  That would still make it favorable but only by $200 ($1200 stimulus less two lost $500 credits).

I appreciate your thoughts.