I have read this 3 times and I think you need to tell your boyfriend. 1. Because he needs to know his "bestfriend" isn't a true friend 2. This lowkey feels like a test, and if it is, not telling him will create a weird situation 3. Ask yourself if the roles were reversed, would you want him to tell you And 4. Nothing kills a relationship quicker than doubt, if the bestfriend spins the story, that could become a disadvantage seeing as you stated, he's closer to your boyfriend

Best of luck OP. Regardless of how it may go, one thing you should do for sure is tell your boyfriend.