Must a girl wear hijab in front of the one who raised her, took care of her, and spent on her, even if her doing so is considered ingratitude and a denial of his kindness?

You must accept and implement the Sharia ruling. If there is no mahram relationship between you and your guardian, then you must wear the hijab in front of him. The arguments presented by this person are incorrect and contrary to Sharia. His kindness will be rewarded by Allah if he is sincere. You must explain to him that observing the hijab is a Sharia obligation separate from his kindness.