Is it advisable to do exercise when affected by Dystonia Disorder? Which activities would you suggest and how intense should they be?

Yes you can't get any exercise is the air but he will have to do skip stretchy and try to relax I'll relax Dystonia

Walking and other light exercise is certainly advisable.

Mine is generalized dystonia, (but mostly stays in the cervical area). Every time I've tried PT it's caused severe spasms. ANYTHING you do, the most important thing is to take your time. Many of us "dystonian's,"(just like that name over "dystonics"), have difficulty with movement that requires focus. Dancing and swimming are great bc you move any way you want/can and requires no counting or focusing. Distraction exercise seems to work as well. That just means doing a few different things at once. You need someone that can tell you different things to do to keep your mind from over focusing on one thing. Like playing doing pilates and then they challenge you to rock, paper, scissors randomly. Several random distractions. It's a neuromuscular disorder. We have to learn to trick our brains into functioning properly with our muscles.
Like everything with dystonia, take YOUR time. We are all different, that's why they don't have a standard handout to take home after physical therapy. I was told for the past few years not to do PT. Dancing and doing random crazy things around the house helped.