Is it permissible to marry a girl to whom I am betrothed, then I discovered that she is acting as a go-between for her friend and a person who is connected to her friend and who is also my friend, and she considers him a brother and asks him about me?

What are called friendships and love relationships between the sexes are not from Islam. Therefore, it is not permissible to mediate between a young man and a young woman to mend the relationship between them, because that is cooperation in sin. Rather, they must be advised to cut off that relationship. The girl's claim that the non-mahram young man is like a brother to her is false and an embellishment from Satan for disobedience. She must treat him as a non-mahram, by avoiding seclusion and unnecessary speech. If she does speak, it should be without softness in speech. The questioner must cut off his ties with the girl until he marries her.