What type of advice would you give someone who believes that changing is useless or there's no point in it because they've done so much sins in the past? I've tried presenting the examples of Aba Thar and al-Hurr but it seems like he's not convinced

One should not despair of the mercy of Allah. Allah is the Oft Forgiving (al-Ghaafur).

Check this out: https://www.al-islam.org/greater-sins-volume-1-ayatullah-sayyid-abdul-husayn-dastghaib-shirazi/second-greater-sin-yas-despair