I think he means to prove she lies at will, to enhance her image. He is vehement in his statements that she did not contribute at all to her school or college expenses.
She claims she did not have any knowledge of, nor approve her friends releasing confidential information in her letter to TM, so as to neutralize all the press stories about her ghosting her father. TM asserts that she knew very well what said friends intended, and approved them defaming him to People magazine with the sole purpose of improving her image with the public.
She has done this before when she solicited JM to act on her behalf to obtain a more favorable article about her.
Duchess of Sussex asked friend to try to influence story, publisher claims
DM defence is that her actions in discussing the letter with 5 of her friends who at her request then acted on her behalf, leaking the contents, invaded her own "privacy" voluntarily.