Friendship, like any other relationships, has its ups and downs. At one point or another, friends would have to experience some troubling times that would challenge the strength of their relationship.
Most of the time when people are angry it is something within them and they use everthing and everyone else as an excuse for being irrate all the time.
"The things you claim you don't like about other people are really the things you don't like about yourself".
Reality Check! Think about it and be honest with yourself and help your friend to see the light.
Understand that anger, just like any other emotion, is a choice and maybe you can help your friends choose positive reactions instead of negative ones.
Encourage them to focus on the good in themselves and everyone else and not be so mindful of imperfections and flaws.
if you can be patient and foster positivity your friend just may follow your example.