How would he know this is the time to stop working for others and start his/her own business.
When you have The Top Idea in Your Mind. When you have a concept of something you know you want to do, understand why you wish do it, know why it will be successful and that you can pull it off. Not before you have a clear vision of what you intend to do. In fact that burning idea is what should trigger your enterprise.
The second part of question is, if he decides to role own company, how he should take a start?
The most important message has been given to you by Pierre. I'll second that. Quit your daily job when it comes to starting the active work on your venture.
Idea elaboration, analysis, research, experimentation, even UI mockups, you can do it besides your daily job, you will have about 2-3 hours each evening. It's hard but with strong determination you can stretch your muscles and keep going during the initial "preparation" phase.
The real issue is, when the concept is ready and you need to start coding, it's very hard to do it as a second activity. There is a lot of work to be done and it's very demotivating that you can only advance in small steps, being forced to interrupt your coding sessions because of going to bed.
I used to live like this for about two years coding various stuff. I used to come home at about 6-7 p.m., have a short dinner and start coding until my eyes closed. It exhausted me. Hint: avoid supper or eat little and no meat/fish otherwise it will make you sleepy.
Last year I was fortunate enough to get laid off (the company outsourced the dev department). I saw in it an opportunity and decided not to go fulltime for the time being. I've been developing my little project since January and the progress I've made has been astonishing. I never knew I could progress that far in that short time just coding alone. I seriously doubt I would have been able to be there with evening work even in a couple of years.
I however enjoy being able to just live home without having any noticeable expenses or bills to pay. If you don't have that opportunity you must save money to stretch over at least a year.
And finally, don't be discouraged by the thoughts that all niches have been taken and you won't squeeze in. Avoid rational thinking. Don't let a Vulcan in your head talk logic, just believe in your cause and go for it.