I am making the assumption that you have a physician’s okay and that you are knowledgeable about possible diseases that are often correlated with morbid obesity. I also assume that you have – as well as is possible – did a screening and therefore have a rough idea what the person can and cannot do.
My first recommendation would be aquatic exercise if a pool is accessible.
If it is not, I would begin with strengthening, particularly the lower extremeties. Very obese people often have painful knees because of their weight which in turn prevents them from doing any cardiovascular exercises. What you can do as exercise may initially be very limited, and if the person is very deconditioned, you may encounter painful muscle cramps with even light exercises.
I found the first few weeks to be marked by trial and error because you will probably encounter limitiations you were not aware of (and neither was the client).
At times, just some assisted stretching may be helpful. I always tell people in that situation that the training has to be interactive and that you have to rely on their candid input to see what works and what does not.
I have a bodywedge which I like to use because a regular bench is often too narrow and therefore scary. Fortunately, I also have a Feldenkrais treatment table with is wide enough to be done for floor exercises if the client cannot get up from the floor. I put the Bodywedge on that table, and that works really well.
I wish you good luck with your client. You can only make a precarious situation better.