Yes, if one doesn't have any choice but to sit and pray they should sit and pray.
Now, there are two scenarios:
a. The person is sure (100%) that he is not going to get cured: Then it is permissible for him to pray qadha in a sitting position.
b. If he is not sure, then he is not allowed to pray qadha in sitting position and will have to wait until he gets cured.