My advice in this situation would be to talk to the founder and ask them to hire someone else and let this guy go. If they won't, you should probably just quit

My advice in this situation would be to talk to the founder and ask them to hire someone else and let this guy go. If they won't, you should probably just quit. You will be carrying this individual for a long time, and it may take a third or fourth person coming on before it becomes more obvious that this individual is a problem. In situations like this, the founders should have hired on a technical lead and have both of you reporting to the lead. Or one of them should be technical enough to be the lead. I would suggest looking for opportunities that are managed in this way in the future.