Yes, it kind of sucks. $10k is not enough money to waste goodwill on or potentially voiding your reputation.
Good luck.
The $10k won't change that. Your cofounder leaving makes it even more likely your startup will fail. And that means that should the startup fail, your startup will have failed and you will have done the wrong thing.
On the other hand, if you disclose the status to the investor, the worst thing that can happen is that you walk away from a troublesome project with a potential future investor who sees you as a person of integrity. The best thing that can happen is exactly the same as the best thing that can happen if you don't disclose. And the odds of that best thing happening are improved because you don't have to waste energy on a web of deceit and because you have an ally who will talk well of you.
A person is only as good as their word.
I think that's a shitty outlook. Of course your startup has odds of failing, but if you quit on your idea because of this hiccup, your chance of failure is 100%. These things happen. Be 100% up front with your investor, tell him you need some time to find a new co-founder, and then go find one. If the investor can't stomach this challenge and wants his money back, all good - his money wasn't worth the hassle of having an investor with a low risk tolerance. But perhaps he's someone who either believes in you or your idea, and would want you to persevere and get this thing off the ground, not jump ship at the first sign of difficulty.
Your other options are to hire some freelancers/developers to help you take your early stage prototype to a public-facing finish line. Or learn to code yourself, that's always an option ;)
I don't think brudgers was being negative, but rather saying something like, "this event doesn't suddenly change the most likely outcome of your startup, which was, and still is, failure."
This is an opportunity to fail or pivot with integrity (or, of course, to stay the course and find new talent).
Even if every person really does have their price, $10k is pretty low to sell one's integrity. At least among anyone I'd care to associate with.
That's not entirely fair. If the investor had been told there was a team in place, and relied on this in making an investment decision, a co-founder leaving is a material event that the OP has an ethical if not legal duty to disclose. This is especially true since none of the money has apparently been spent.
Why losing a co-founder is a problem? Only they have the technical ability? Only they have the insight to the sector? The fund depends on their name? The fund depends on the partnership? co-founder is irreplaceable in some way? Op has time limits? Op lacks the technical/social abilities to replace partner?
HN is a good place for looking for talent, but you need to give more information about what you want to build. Also you need to create interest in people because it's really an early stage venture with low funding. Considering money is the first incentive you need luck, or you need to show that you have the chance to become a unicorn.
Also you may try angel.co.
So good luck.