I'm not going to sit with my friend at lunch.
Why don't you want to sit with your friend during lunch?
I want to see how sad she will be when I don't sit with her.
Ignoring your friend is rude. I don't understand the point of wanting to be rude to your friend.
It's not rude, she can sit with someone else if she's feeling lonely.
That is a cruel joke that serves no purpose. She may think that you don't like her anymore, and the friendship will be ruined.