My bf has a stalker,what do i do?

i have a friend, and her cousin is stalking my bf.she calls him every night and when i confront her about it she always denies it,one night my bf put her on loud spearker so i can listen to the conversation and its obvious she is interested, what do ido? i dnt want to hurt my friendship with my friend cos i know just how much she loves her cousin...and i dnt doubt my bf"s love for me and i trust him but i dont like the idea of him talking to her every night,he tried several times to stop her from calling,even tried avoiding her calls and she started phoning in the office line.he also tried being rude and cold towards her but she just doesnt stop,pls help what should we do?