Is the husband considered sinful if he does not pay for the cancer treatment of his divorced wife, even though she owns gold that she can sell for treatment, and his financial situation is modest and he has another family? And is he sinful if he demands the gold back that he bought for her during their marriage?

You are not obligated to spend on her, and you do not sin by not giving her money for treatment or general expenses. You also do not sin if you refuse to give her anything or if you ask her to pay something from her gold in exchange for your spending on her. However, your voluntary giving in this regard is an act of kindness for which you will be rewarded. If, however, she is pregnant or wishes to spend on her children, you would sin if you refuse to do so, and it is not permissible for you to demand anything from her in return for the spending that is obligatory upon her.