My Friend Is Being Blackmailed At Work

My friend told me he was worried about this girl at work after seeing something he shouldn’t have. He works at a hospital and I guess they have unisex bathrooms. He went in early for work and caught one of his female coworkers using an adult toy while trying to head to the bathroom. She saw him and instead of feeling embarrassed or worried, she asked him to join her. He declined as he was weirded out and has a wife and child. He was worried she would go to someone and lie as it would be her word against his. Well, a few weeks later and he told me that in person she threatened to tell someone if he didn’t meet her up for “lunch”. I asked him if there really isn’t anyone that he can tell, as I’m sure he doesn’t want to lose his job or his family over this. I suggested he recorded their conversation so he can at least have some sort of proof… but is there really nothing he can do if she lies and they believe her over him? She doesn’t text him or anything so there is no trail or evidence so far of what she’s doing.