What do I do?

My best friend and I work together in a restaurant, she joined after me as I introduced her to the job. We are both 16 but she could easily pass as 18/19.

One of the chefs who is over 35, has been messaging her constantly and they text all day everyday, he always says to her at work “I need you” and weird creepy stuff, he constantly asks to mert her and tonight I found out she met him and they smoked weed together and also kissed and he tried to do stuff with her.

What do I do? She made me swear never to tell anyone but surelt this is wrong a grown man over 35 with a 16 year old?