I wouldn't have sent her anything in the first place.

Your reaction seems disproportionate... It feels like children being upset because their 'best friend' was talking to another child. This woman is an adult choosing not to attend another adult's party. That is her right. So, I would let her be.

Not every person or group of people you meet will be your best friends. There are casual friends, there are acquaintances, or people who can be friendly but not overly invested. There is no need to be so upset if others don't fit your definition of 'real' friend.