Setting up a bicycle for someone with restricted knee articulation
Look at Power Cranks.  They are a crank set with the ability to freewheel independently of each other.

It allows a slow build to fully articulated movement if additional rehab is still a goal, and using each crank independently means using a bike is still possible, even if doing so uses a pedaling motion which would be awkward for someone who had an option for full movement.

They also have adjustable crank lengths on each independent arm, which would allow you to use a shorter crank on the leg with the limited movement.

They are often used as a rehab tool, are a real, readily available, and not ridiculously priced product, and are install-able with normal bicycle tools. 

One caveat: They are shifting to a new poly carbonate fixing cap. If you order, I recommend requesting the old alloy fixing caps. The last set of poly caps I installed broke easily.

FTR, I have installed and used this product, and have no relationship with the manufacturer, nor will I be compensated in any way for this post.