Need advice on a guy i like

(lets js call everyone in this A or B instead of their actual names) ok so i think A likes me but he's in a relationship with B. B is one of my best friends but i really like A. B doesn't even like A that much and i really like him. A flirts with me a lot and he litterally said that he does like me but he still talks about how much he likes B. I want ro tell A that he should break up with B if he's gonna talk to me like this and stuff but he should stop talking to me if he's fonna stay wit B. B has also kissed other people and stuff while dating A. Sorry if i got the letters mixed up durinf the story but i dont think i did 😭