Oh I have been there. It was in HS but I have been through it. I think you should protected yourself as best you can...here is my story..hopefully I dont become long winded..If I do bear with me.
My Best Friend(BF) and I had known each other since third grade. She had moved down the street from me. She was also in my class, so naturally we got to hanging out. We never spent a day or moment apart. We still have a cache of movies from the past that we ''QUOTE'' from. Anyways, We stayed this close until our Sophomore year(we didnt have a middle school setting, so the first year of HS for us was our Sophomore year). She was more excepted by the popular crowd. She got into partying and I got a boyfriend. We had just become to different people. I was broken, oh so broken. She was hanging out with a few girls I knew pretty well from Junior High. They were horrible people. Reasoning with her at that point only drove her further away from me. She tried making ammends with me my Junior year of High school. I was already so bitter that she had abandoned me when I needed her the most. I did decide to just ''keep in touch with her'' We would say hi when we saw each other.
She went off to college...She was away most the time. We didnt really talk much during this time. I still saw her family every once in awhile, so they would keepme updated on how she was doing. By this point too I was not bitter, more sad we didnt have each other anymore. Taking a break from school for a while she went to Africa, to work with an all girls orphanage. She did some pretty amazing work. The photo journals she brought back with her were just amazing.
By this time I had been married and given birth to my oldest. I rememeber going to her welcome back party. She was living in Seattle going to SPU. I dont know why, but ever since she came back from the CONGO(the main place she was in while in Africa)we have been attached back at the hip. She is a full pledge AUNTIE in my kids. She is the ones person that I honestly could never be mad at. she has taught me what it means to over come stuff.
My point to the rambling is if you decided to protect your heart from the "OTHER'' in your friends life, I wouldnt blame you. But do not banish your BF for life. This friendship she has with the ''Other'' maybe short lived. If you still want to see each other everyonce and a while still see her. If she is truly a BF she will understand. You have to look out for your own heart first before you can take on others:)
Good luck mama! know she stills loves you...you still love her. You just need to step back from the situation for a bit.