If their idea was so easily replicated then they probably shouldn't be creating the business in the first place.
But, just for fun since some people can't answer the question, let me throw some curve balls that might entice someone away from their current startup.
1. Maybe you're approached by a stealth startup in the same space as you, but still different. Their approach to the same market is one you hadn't thought of and you can see the value of the team behind the product and your addition strengthens your over-all endeavor.
2. The person approaching you has a huge network and an interesting product idea. You can replicate it, however, you know their support network and media connections can help drive the success of the application.
3. The person approaches you with enough money that you are willing to give up your product idea.
4. The person offers you a salary and although you can replicate the idea on your own, the salary acts as a way to cut your risk of engaging a new startup.
5. Not all developers are visionaries. They may be able to take an idea from a simple concept and write an application, but it takes more than a well developed application to make a successful product. Industry insight, market expertise, sales experience, these are all valuable skills that drive success.
Feel free to comment if any of these would put you over the tipping point.