What should I do when my best friend seems to have fallen in love with my boyfriend?

You should sit down and talk to your best friend, you always need communication in any kind of relationship. Whatever your best friend tells you, you need to take that information and choose what you will do with it

The only thing you can control is yourself, so it is a waste of energy to try to control anyone else's feelings. Being aware and honest with how you feel is important for your health and these relationships. Talking to your best friend and your boyfriend with compassion and honesty may be helpful. How does your boyfriend feel about your best friend? If you and your boyfriend want to have a monogamous relationship, it may be helpful to have boundaries that will help you continue your relationship with your best friend and also have a relationship with your boyfriend.