My friend caused me to get a bad grade so I'm not going to put any effort into out next assignment.
Why would you not help her?
She shouldn't have done that to me
It's okay if you don't want to help someone who let you down academically. You should focus on your own studies instead so you don't have to be let down by them. That way you can be sure you do well on tests.
It's not about me being let down by her, I just don't want to help someone that cheated on me with my ex. How could she do that to me?!
Ok, I understand why you are so angry now. Rather than purposely hurting both of your grades to get revenge on her maybe you could talk to the teacher about changing your partners.