First of all, she should be able to understand that she inherited sin from the fall of Adam and Eve. In Romans 5:12 it says “12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:”
This sin would make us unworthy to receive Jesus’ body and blood if we are not absolved from it. In 1 Corinthians 11:27-29 it says “27 Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord.
28 A person should examine himself first, and in this way let him eat the bread and drink of the cup.
28 But let a man examine himself, and so let him eat of that bread, and drink of that cup.
29 For the one who eats and drinks without careful regard for the body eats and drinks judgment against himself.
29 For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord’s body.”
Therefore, in order to be free from the bondage of this sin and be worthy of accepting Jesus’ body and blood, one must be first baptized.