In which case the responsibility falls on the boss. And the first thing they need to do is get a lawyer. Firing someone without good cause ("I fired them both after she accused him of sexually harassing her") is a BAD idea. You're opening yourself up to suits. The situation, the investigation, it all needs to get documented and reviewed by people who know this stuff.

Once a startup goes beyond some friends/colleagues tinkering in their garage or bedrooms, you need to retain proper staff to handle your various employee situations. You need people that know how insurance works. You need people that understand retirement plans. You need people that understand legal obligations re overtime, travel compensation, etc.

With a small company you can conceivably get away with a handful of lawyers on retainer, or one primary in-house counsel who consults with specialists as needed. But you can't get away without having lawyers and HR people in the long run.