One thing that you guys might be interested in: there has been some limited investigation into tandem cycling as a therapy for Parkinson's, though it's somewhat anecdotal. What research there is suggests that it's about being forced to cycle at 80-to-90 rpm at a lower resistance which still keeps your heart rate up, as opposed to cycling voluntarily at whatever pace you want while being coached to keep your heart rate at the same level: 5 people who were forced to cycle at the higher RPM showed dramatic improvements in symptoms, as opposed to 5 who set their own RPM with coaching.

Since those numbers are so low, there is right now a team at the Cleveland Clinic which recruiting for a much larger study (100 people total) that they're calling CYCLE: 40 people with PD will do forced exercise on a motorized stationary bike in the lab, 40 will do voluntary exercise on a normal stationary bike in the lab, and 20 will not be directed to exercise at all in the lab:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4415238/

This is right now still in the recruiting stage so it probably has a couple years before it yields results; but if your friend can't live in Cleveland for a year, you could try to find another way to keep yourself at the 80-90 RPM levels, whether tandem biking with your friend or else by jury rigging a motor or perhaps even self-discipline.