My best friend is annoying me what should i do?

i love my best friend, but she has been facing ual advances from this guy who she does not like. she doesnt like it, and she sort of told him to back off, but she did not do it well and hell start again soon. ive given her loads of advice, but she wont take it, or she says she will, and then she doesnt. i know she is not happy, and i want her to do something about it, because she is not in a normal situation, and she is too indecisive to do anything. if she does end it, she says theyll still be friends. Not normal! what should i do? im considering talking to the guy myself?