My boyfriends co-worker is kind of . . . promiscuous, what should I do? Am I over reacting?

Ok so recently my boyfriend hired a new girl. She has two kids and lives with their father (her bf of 9 yrs). She recently lost a lot of weight and constantly talks about it. She lifts her shirt up to "fix her pants" all the time and she started sleeping with a married man at the office. I don't know what to do or say!!! Help!