Because he didn’t want her to quit talking to him.
He lies because he is a liar leading a double life.
He can’t be honest about where he was because he’s afraid she’ll judge him harshly and quit being friendly with him!
He got upset, because he’s trying to balance the illusion that he has a boring life vs the reality of him out screwing around.
She’s going to find out eventually whether you tell her or not. Life is funny like that!
Expect him to “freak out”!