So, it sucks, but it happens, and it won't matter nearly as much in June 2014 as it seems to right now. Your co-founder will, hopefully, not find out about this through guessing "Is this my company?" on Twitter, but rather in hearing it from you. You may be heartbroken about it, you may cry a bit (totally OK). Then, you will put on your Responsible Adult pants, call your employee into the office, and tell them the si…
And one more thing: Don't wait for the 30th day. Do it tomorrow.
Talking to your co-founder "sometime this week" and your employee because if you don't fire him you "fucking won't make payroll" for all of two paychecks should really have been the first moves.
But then, this doesn't sound like someone who cares about much else other than deflecting failure onto his investors and co-founder.
No need to be defensive about it if it did not work out, just go talk to your people.