Messing around with my bestfriends boyfriend?

you didn't do anything, you just talked to him, so she really got nerve accusing you after what she once did. I'd say, if she keeps nagging you about it she's not gonna be worthwhile as a friend. if you want, talk to her about it and state the truth - that you didn't do anything wrong. if she's really your best friend you guys should be able to work it out. though if you do decide to break the friendship, and you feel hurt, you could always sleep with her ex as a final revenge, providing that you are that kind of person.