Work Place and Social Media Rants of Employee

An employer, while using computer at the office, found that last employee who used the same PC left his Facebook still logged in. employer just ignores this, at first. What caught his attention, though, as desktop notifications kept on coming in was a notification of private message coming in from a recently terminated employee.

Upon reading the messages, the employee made mention of "low morale" in the office and that he and another colleague are considering offers from another company. And it's not about being passionate about work but money.

Can the employer confront or discuss with the employee regarding these PMs? Am I right to assume that the employee cannot claim violation of his right to privacy since: (1) it was his fault that he left his FB account logged in; and (2) employee was using company's PC and network, in the first place.

Hope anyone can help me sort this out.

Yes the employer can confront - but not because of the content but because of any policy regarding facebook and social media usage.