best friend help!

Joanna
My best friends boyfriend has been stringing her along for three years and telling people he wants to break up with her but then not doing it! I try to let her make her choices, she swears he is the one and she thinks he is her forever person, and I try to be supportive but I know it's going to end badly, and I wish I could protect her. Has anyone ever been in this situation before? How can I make this easier for her?