> 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.

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.