My best friends boyfriend likes my other best friend?

Okay, so I recently found out that my best friend's boyfriend likes my other good friend. She's been through alot and the boy (lets call him frank) he says hes going to kick the crap out of the good friend's (lets call her wanda) boyfriend. Well my Best friend (well call her Donna) has no clue about anything of this. Wanda told me not to tell anyone about anything of this. Frank and Donna dont know i know. But I really want to tell Donna that Frank likes Wanda and that he's saying all this stuff. But I dont know the entire story.. and I don't want her to hate me because I didn't tell her sooner!! What do I do!?