You could bring her with you and leave her in the living room while your "friend" gets to "hang out inside of mommy." When she asks what you mean, put your hand on her shoulder, look her in the eyes, snd say, "you'll understand when you're older."
Then you put a bunch of red stains on your clothes and face, like ketchup or red food dye, and make sure she never sees any reference of him ever again. Tell her to never mention what she saw to anyone.
She'll never know it was sex.