Why should I apologize to my friend for calling her a fat pig when she deserved it?

I have this friend who is extremely overweight, but she constantly bitches about other peoples weight. I had my 21st birthday party saturday evening and I invited a friend who is slightly reclusive. He lost his brother a year ago so hardly goes out anymore and has gotten obese after taking comfort in food. As soon as he walked through the door my friend started calling him fat. I explained to her what he'd been through and warned her to shut up. But she didn't so I told her she hardly needed to ***** about anybody else because she looked like a fat pig herself. She ran off crying and now keeps texting saying she is waiting for an apology. Why should I apologize?