I think the cupcakes were most definitely left there for Bianca. If you relisten to her visit with the doctor at the beginning she mentions sometime about Karen Carpenter. If you look up ipecac syrup on wikipedia you find out that overuse thereof can lead to death. One such famous case was that of...Karen Carpenter....
Then she eats an ipecac cupcakes and gets murdered. It just seems a little too coincidental to not be put there on purpose.