What is the best way to confront a cheating ex-girlfriend?

A guy just found out that his recent ex was cheating on him, and has decided to confront her. What is the best way to do this? What would you advise him saying and doing?
Angrily let her have it
Calm but firm let her know she means nothing
Something else. Leave opinion in comments.
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What is the best way to confront a cheating ex-girlfriend?
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