If you're having trouble with one particular idea, I'd suggest taking a step back.
There are two ways of building a business. The product-first way starts with an idea, then you build something, and then you try selling it to the market. That's what happens on ProductHunt many dozen times a day. And most of these products flop. They're solutions looking for a problem.
The audience-first approach, however, focuses on finding your ideal target audience first, making sure it exists, and then learning about their problems and challenges by embedding yourself in their communities. From there, you listen, observe, and look for interesting problems to solve. THEN, and only then, do you start thinking about your idea.
It's much easier to get someone excited to help an existing group of people. If your friend has a hard time committing to your idea, take that birds-eye view together and commit to "spending two hours together, analyzing the existing problems of one particular community." Maybe even better, spend two hours brainstorming all the communities you can think of, and find one that both of you would love to serve and empower. From there, look into the problem space, and come up with an idea together.
The best ideas are CONSEQUENCES of this process. They are also the most validated, as you already know who you'll be selling the product to, and you're sure they have that problem — because you found them talking about it in their communities!
If you can do this together, you'll both be equally motivated.