Because if he does not have a witness or alibi, all she needs to do is have someone else say they were at the party, or saw him there, which is all she needs to do to change the narrative by proving he lied about being there.
She has no one. The best she can do is have her HS pout togetger a letter saying we believe her. Those who signed it are from classes of 1967-2018. This is meaningless. The people who signed the letter were not even in school with her or knew her.