How do you stop a "special friend" from wanting to **** your husband?

I am a stay at home mom of 2 and my husband owns his own company. A 32 year old helpless tramp works for him. She calls him on his cell all the time about her personal life. I have asked BOTH of them to stop and that was over a year ago. My husband insists that she is just a "friend". Then why doesn't she call our land line? Why has he been to her apartment...behind my back? Why have some of the lady staff members gone out of their way to approach me about my husbands behavior at the grocery store?? I love my husband and we have a wonderful life, but how do I get him to STOP being emotionally involved with this helpless tramp?