Can God forgive a girl who lied to a boy who died, and she is repentant and remorseful, and will God conceal her and not expose her in front of that person?

You did well by repenting to Allah from the forbidden relationship with the young man - may Allah have mercy on him - and you owe him nothing. It is not permissible for you to be preoccupied with his matter. Rather, you must realize the sincerity of your repentance, reform your heart and deeds, and be keen on obeying Allah. Repentance erases what came before it, and one who repents from a sin is like one who has no sin.