So I can't speak to his particular situation. But in general I disagree with this. I think quitting the job is the most important part. Consulting is the safety.
"Leap and the net will appear" - John Burroughs
"Leap and the net will appear" - John Burroughs
Consulting requires networks and connections. If the person stick too long as a "Senior Software Developer" and rarely meet non-geek human being (potential customers), it'll be a while before the person can get a well-paying less-stressful consulting offers.
Second, if you work in the technology field, meeting geeks is a great way to find potential clients. People in the technology field tend to move around a lot from company to company and from position to position. So the developer you meet at one company today may well be a manager or even a CTO at another company tomorrow. They will probably be the very people who'll need to hire consultants at some point in the future. Make a good impression on them and keep in touch, and you'll have a nice network of contacts for consulting work down the road.
As a senior developer, I bet he's already made plenty of such contacts. Now it's just a matter of getting in touch and letting them know he's looking for consulting work.