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.
That is assuming everything will go well: this person will get a few customers ASAP, within 1 week or 2, to stay a float for at least 1 - 1.5 years.
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.
First, if he quits he might be able to consult for the company he just quit from. They, presumably, know he's good and can do the work. So they could hire him back as a consultant (where he'll probably make more money and perhaps do more of the kind of work he's really after).
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.
Agreed. We can some how manage to make our living on consulting and cutting expenses, but if you are still in day job, it will take up major part of your time and energy. Also, some companies will have employment agreement that you will not work for any other thing
"Leap and the net will appear" - John Burroughs