Don't stoop to her level.
And yeah, her threatening your crush shows you exactly what kind of person she is. A pig - and that's an insult to pigs.
By the by - if you want to find out who is tattling to her - think up some plausible but not true(nor harmful) stories about yourself. (DO NOT TELL anyone even your best friend what you are doing.) Let the different suspects have different tales and then see which one she ends up knowing about. That will let you know who is reporting back to her.
And good on your best friend from keeping you from assaulting the moron. (Can't use the words I really want to use!)
If you want to do a sculpture - take it outside Don to do the work. Pauses - just what kind of sculpture?