I think it's important to understand your business goals before jumping into app development. Have you created a Lean Canvas or something like that? Do you know who the customer is and what they want? Has the developer built anything that they can show, even a mock-up or prototype? It's tempting to jump straight to building your idea, but I would caution you to think very carefully about the product/market fit first, doing some customer development, before you build too much. And, if you're on board with that approach, it's a great way to start the conversation with the developer, in terms of their willingness to also validate your assumptions before writing too much code.

I would try and get a warm intro.

If you can't, then email is best.

Just say
- I love the app
- I've been thinking about this market for a while
- I've actually [insert credibility - past work experience, research, access to customers, etc]

Then ask if they'd like to get on a call to share notes?

Going in all excited and mentioning that you think he's not focused, etc might turn them off right away.

Build the relationship slowly, as it's too easy in today's world to archive emails and never respond.

Hope that helps. Call anytime to discuss further.

Dan & Sam's answers are solid. I'm going to add some more context for you specific to your question. Often, solo developers don't want to go it alone but don't know where to start in finding a suitable partner to assist with marketing, biz dev, fundraising (if required) and the like.

So the fact that you have found either other and both have interest in the same space, suggest that you could be effective partners, if you can confidently convey why you are the right person to execute the non technical aspects of the business. As Dan suggests, don't go for the hard sell. It's primarily going to come down to whether you both like and trust each other, so work on establishing that common ground.

Sam is right in that if you haven't been rigorous with customer development, there are many cases in which failing to do customer development and empathetic research prior to shipping the product wil cause big problems but there are also a few exceptions to that rule.

Happy to talk to you about how to build the relationship as well as how to shape the product in a collaborative context