Facebook could prove our employee lied! How far can we go to gain access?

Q. One our employees called in sick for a shift during a recent holiday weekend. He told several co-workers that he didn’t come to work because he was having so much fun at his cabin. According to a few co-workers, he made several Facebook posts about his various recreational activities on the day that he was allegedly too sick to work ... Can I ask one of the co-workers to show me the Facebook posts?