If it happened a few months ago, then it obviously didn't take 12 months to write, so throwing in some over-time and rewriting it shouldn't be that big of a trainsmash - obviously re-using bits and pieces of that software that can still be salvaged.
If it's completely unsalvageable and the developer who caused the mess made a mess before, I would seriously consider getting rid of that developer for the sake of the startup, startups can't afford one developer keeping the startup back.