guy threatening to commit suicide

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my friend is a real jolly person...but from the past few days she can hardly sleep... she was going around with this guy for two years...during this time they were physically intimate...the guy used to mentally harass her a lot..she tolerated it initially but could not take it any longer and dumped him... he now threatens to commit suicide if she does not go back... he says he will leave a suicide note holding her responsible... she is really worried... what should she do??