Should I let on that I know their secrets?

I overheard my man talking to my best friend on the phone. He offered to come over to her house and perform some "boyfriendly" type duties. I know her bf is a creep and she's desperate for a new purse....but does she have to go after my man? She's just the type of skank to do something like that, and she's been talking about getting a new purse for a long time now. Should I let them know that I know what's going on? Or, should I let him proceed and then ask him for something new too?