What is my boyfriend cheated on me with his girl best friend?

I am sorry to hear that your boyfriend cheated on you with his girl best friend. Cheating in any form is a breach of trust and can be very hurtful. It's important to communicate openly with your partner to understand why they chose to cheat and whether they are willing to work towards rebuilding the relationship. It's also important to prioritize your own emotions and well-being during this time, and seek support from friends or a therapist if needed.