By passively accepting this situation your are part of the problem. Morality aside, your coworker that denigrates employees by calling them girls is opening you

By passively accepting this situation your are part of the problem. Morality aside, your coworker that denigrates employees by calling them girls is opening your 15 person startup to justifiable lawsuits (discrimination and hostile work environment) that could easily shut it down. The fact that the main problem source is friends with your boss makes it a more challenging conversation, but in no way alleviates your responsibility of having it.