I don't know why people here think

I don't know why people here think it's so ok and normal to back out of a signed offer... Sure they can't do anything, but you are doing something really unethical, and really burning your bridge. And in your case, your friend is involved too.. sigh... This is so bad in so many ways. If you insist on backing out, I suggest you come up with a REALLY good lie, and really make it up to your friend.