It is not okay to ruin other people's friendships.
Carl dislikes his friend Jane's other friend Mack.
Carl wants to spend less time with Mack.
Carl declines invitations to hang out with Jane when Mack is invited too.
Carl tells Jane that Mack did horrible things to get her to stop being friends with Mack.
Carl stays friend with Jane but does not have to see Mack.
Jane finds out Carl lied and stops being friends with him.