Some parents or suitors choose to perform istikharah for marriage proposals, and that is their choice, and if they completely depend on the result of the istikharah, it should be respected.
As the girl's father is her wali amr, and he turned to istikharah, their decision is going to be based on that, even though your istikharah turned out good.
In this situation, you can proceed by having a conversation with her father, and trying to find ways of either doing another istikharah, or negotiating what would be best for both families. If he stands firm behind the result of the istikharah, then you must respect that, wish well for her and her family, and pursue another case for marriage.
Also, please bear in mind that istikharah being good for a marriage proposal does not mean the marriage will be perfect and free of problems.
With prayers for your success.