...how has she "abandoned" her friend? According to the letter, she still spends time with the friend, but her friend complains that the boyfriend is along. I acknowledge that its not appropriate for her to bring him where he is not invited, but it sound the friend doesn't want him around at all. Note the clue that the friend rarely dates, so she likely doesn't appreciate when two people are in love and devoted to each other.

I admit that the LW could probably do better at not being joined at the hip to her boyfriend, but she shouldn't be joined at the hip to her best friend either.