You shouldn't hang out with your friend's ex.
Mooz's friend is unavailable to hang out so Mooz is running through a list of contacts, including his friend's ex.
Mooz wants to decide if he should call his friend's ex.
Mooz decides to wait and ask his friend when he's feeling better.
Mooz calls his friends ex and they hang out.
Mooz is relieved that there won't be a conflict.
Mooz's friend finds out and the relationship suffers.