Here's what you do:
Befriend her boyfriend and make sure to hang out with him a lot while she is around.
While you do this make sure to always look suave. Wear amazing cologne, and make sure that you look better than her boyfriend. This will work on a subconscious level to make her associate you with sexyness.
Always bring gifts to her boyfriend and make sure she sees the gifts; but make sure they are manly gifts, you don't want her to think you are gay. Try stuff like a six pack every now and then, or maybe even a bottle of cologne that you wear. This way, she will associate his smell with yours, and think of you even when you are not around.
Now that you have become friends, they will not be suspicious of you when you are around their house. Here's where the plan comes into action. You need to start sending her threatening mail and emails, maybe even sms's from a new number; don't use yours for obvious reasons. When this has been going on for a while then she will obviously tell her boyfriend about it etc.
Bide your time while still setting up the tone of stalkerishness. You see, you are going to frame her boyfriend for the dodgy messages. You need to plant these things in his house, like the same pens used to write the letters, maybe at the last moment a sim card that you used to send a creepy message.
Then, you set it up for her boyfriend to have an 'accident' *nudge nudge wink wink*.
It's all smooth sailing from there.
You act devastated that he is dead, as of course he is your friend; then you offer to help her clean up his things and conveniently uncover the incriminating 'evidence'.
By that time she will be so sad and confused that her mind will latch on to you as her saviour and protector, and that way you have her for life!