Is a person sinful if he does not answer his close friend's questions if the friend only asks about him out of need, and is it permissible to cut ties with him, and what is the reward for continuing to answer him?

A Muslim is forbidden from shunning for more than three nights. The estrangement is lifted by greeting, according to most scholars. Therefore, it is not permissible to shun your relative for not visiting you during your illness. You can make your relationship superficial by merely greeting him, and you are not sinful for not answering his questions. However, it is better to treat him kindly and continue answering him, for that is a great good and its reward with Allah is immense.