Address what the worst case scenario could be, in most startups that would be going out of business. Highlight that it would be a better experience in the same period working for a startup rather than said current company.
It could also be financial, maybe he/she has stock options & trigers that would vest in a few months.
Throughout my life I have taken many opportunities, and not all of them went well. But the only ones I truly regret are the ones which I didn't take.
I don't think that "someone who isn't brave enough to take the risks" is really someone you should want joining your startup, personally.
Woz also probably wanted to build stuff for himself that HP refused to take up. So creating Apple was a logical step. I think the real convincing was via the "Reality Distortion Field" ~ http://folklore.org/StoryView.py?project=Macintosh&story...
The programmers I know are pretty good but they also have PHB (pointy haired bosses) and also hate some things of working in a big corporate.
Learn to develop one. It doesn't need to be as good as Steve Job's, but being able to persuade people that you're onto something is a pretty essential skill as a start-up founder. In fact, I wouldn't start up unless you are able to convince a few people that your proposed business is really good and worth backing. Consider it a first hurdle.
If you can't convince people to join up with you, either your pitching skills are not good enough (so improve them) or your idea sucks (so iterate through that).
And, as others have said, if that person is not really interested in the start-up life, don't push them... find someone else (unless that person is a genius).
DONT offer equity to someone who isn't brave enough to take the risks. I've told prospective hires (and current hires) that providing equity at this stage would cause them more problems than it solves, but then again I'm not looking to cash in my chips in the next 6 months.
Are you a born salesman? Do you have great persuasion skills and killer smile that can motivate others to do what you want? Are you capable of identifying the targets motivation and swaying them over? If you did you wouldn't be asking this question.
Find another.
A programmer is not by default an ideal Startup Founder. Finding someone who is technical and who compliments your skillset is important. Someone who can build stuff. But if someone has to be convinced and not brave enough to leave what they are doing. When things get tough guess who will leave? Are they motivated enough to stick out the marathon? I've often thought to myself, "what is the best measure of a potential co-founder?". Is it a combination of raw ability, skill & determination? Could the "X" factor be a fierce desire to do something for themselves? Build something (product) and profiting from it?
If the fire is not there, a programmer might make a good employee. But Co-founder?
to get people, you need to offer a competitive salary. everyone has to pay the bills. after that, its the perks. if working at a company is fun, who wouldn't want to work there?
To be sucessful at a startup, the cofounders really have to believe in it. If you explain your project to someone, and they don't buy it, and you sell hard and convince them, sooner or later they are going to have second thoughts. And that's going to doom you.
If yes, you are going to need to pony up the cash, have enough to be able to provide for both of you to survive on ramen noodles for 6 months.