I plan on telling my ex-best friends baby daddy that he is not the father.
Why would you tell him he's not the father?
If she wants to play with fire that's not my business, but i'll help her
People are expected to respect other's family business. Trying to convince someone that they're not the father of a child is cruel. You could end up causing a huge fight and ruining their relationship.
I don't give a shit, I'm that mad at her.
It is ok to be honest if you truly do not think that the baby daddy is the father. The baby deserves to meet the father. However, you should not assume this without some sort of evidence as a claim like this could really ruin a family.