I don't know if this will help you I have done the same but not as a close friend for years.
I met this girl she's in a relation ship with his boyfriend for 2 years
took me 3 months to got them separated though cause she really loved her bf.
but I think it will be easier for you.
things I did.
1. Never lost contact even for a day even a simple "how are you?" is ok even if she doesn't respond
2. Don't bring up her bf in a conversation, get personal ask about her sister, mom etc.
3. Sometimes I act helpless and she comforts me, well it's all lies though like **I was completely drunk last night I got mugged** or when she asked "how are you ?" I usually respond like this "I'm ok but I't's better with you around" then get the convo going.
4. watch out for silly mistakes that she make like tripping or dropping a pen; look at her, when she makes a face laugh at her but not in a mean way and just whisper to her "cute or adorable" stuff like that
5. Make her feel that your always there for her even if hell break loose.
6. get intimate e.g grabbing her hands while crossing the streets, when you've been seeing each other for a while kiss her cheeks when saying your goodbyes. you'll eventually end up kissing her lips and do it.
7. Unsure about the results, my result was her bf broke up with her since she told him that we had sex.
my problem now is she still loves her bf and put me in the friend zone
Good luck man !