You can hedge your bets by not going all in on an idea at once. Starting small and seeing how it develops. Having a (part time) job to pay the bills while investing any spare time in her business idea, is a tried and true strategy. I realize this is more or less echoing @swelker101 's advice so I'll expand a bit.

Other ways that compliment this strategy is:

• Create the smallest version of the business that you know will work.

Example: Suppose you want to make and sell furniture but you know the cost versus profit of most furniture isn't that good. However, chairs do really well as they are cheap to make. Your focus should then be chairs with a later expansion into other furniture items.

Side-note: This idea applies to all areas of the business. Eg. you might want a store for your business, but you can start by having the "store" be your garage.

• Slash the budget.

Find the areas in your budget that are luxeries and cut them. If you're living on a part time job while starting a business on the side, taking a hard look at your budget is one of the more painful but ultimately good ideas.

• The Pareto Principle, aka. the 80-20 Rule

I'm a little pressed for time so I'll link to the wikipedia article

https://en.wikipedia.org/wiki/Pareto_principle

And investopedia as to the business side specifics

http://www.investopedia.com/terms/1/80-20-rule.asp

Short version: You want to find and target your biggest customers. Of course, This is more advice for down the line when you've gained some customers.

Also worth reading (And unrelated to the points above):
http://kk.org/thetechnium/1000-true-fans/