"Find another employer" is an advice extremely common advice here on Workplace.SE (see the answer from @sf02 ), as opposed to actually fixing the problem somehow. Changing jobs can be very difficult in many locations / industries. It should not be on the top of the list, it should actually be close to the bottom.
In this particular case: since it seems the company is a very small one, contacting the owners is usually more feasible. Explaining the situation to them will trigger at least some sort of informal investigation from them, maybe even more. Even if the manager in question is a (close) friend of the owners (something commonly encountered in small companies), his behavior is threating their business, which is clearly not acceptable, no matter how well you know that person.
It would definitely help if you went to talk to the owners as a group (multiple employees) instead of alone, it will send a stronger message that this is indeed a serious issue that affects the company as a whole, not just "an upset employee".