beyond the obvious physician’s approval which I assume you have.
Why does an assessment not work? I try to take all my clients through the NASM assessment, often quite modified as necessary but range of motion assessments and static postural assessments are always possible. You can also establish baselines for mobility and measure against those.
Stretching and core work are all very well but overall strength is just as important as the person moves forward. This can be from light elastic resistance to dumbbells. It depends where the client is.
With conditions like that, you have to be flexible because it may be better or worse from day to day. A very conservative start is obviously indicated but I would gradually increase complexity by adding balance challenges.