I wanted to know is there any dua to control the anger of a person. I have a friend who loses his temper on petty issues and regret for it later. I want to help him. Please suggest me what can be done to make him control his anger?

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It’s not only dua that is needed, but following some recommendations such as having wudhu at all times, sitting down when angry if standing up or vice versa, go to anger management classes or to be equipped with knowledge of dealing with it

As far as dua, there is a few, but for example this one from Sahiffa Sajadiya is good:

http://www.duas.org/safeguard.htm