Just stop answering her calls, distance yourself from her all together, (even though you shouldn't have to do this.........) when they go out and *she* wants to tag along, be there too.
Tell your guy that her constant attempts to contact him make you uncomfortable with *her* and her intentions towards him. I think honesty is the best policy in this case with him, with her...I think it's in some women's nature to feel the need to "compete" for attention from *every* male in her life no matter *who* it is, so speaking to her probably wouldn't do anything but aggravate the situation!