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.
What's best in my book is saying "well, we have a prototype and a bunch of customers lined up, a board of advisors, some people close to investing, done enough customer development to know what people need, we just need someone to take our prototype or screens and make it production ready." Then I'll think, "Well I could start at the very beginning, or..."
If you look back in time, you'll definitely find a bunch of non-technical people looking to start technical businesses. Some of them have good track records of churning success out of it too.
Among these "non-technical" people, you'll find that some aren't entirely non-technical and they do understand what it takes to lure their target audience into using/buying what their business sells. For example: I have a cousin, a mechanical engineer, who's always looking to do mobile apps. He doesn't have the skill to develop them himself, but does have sufficient knowledge required to deal with any technical guy and reads a lot (possibilities on the specific platform, understands technical terms like 'Activity' understands that web services called 'APIs' are available for use, etc).
But again there are exceptions, and I guess we've all come across people who just blindly want to develop a GroupOn or Facebook clone, because the original ones were successful. These are the ones I would clearly stay away from by just politely giving them a good reason or try pushing some real business questions into their mind (like "do you have something different to offer?").
Apologies for not being clear earlier.
P.S: That cousin of mine is a mech engg but works for an accountancy firm :)