Is it permissible to perform prayers on behalf of a deceased person who regretted not praying and intended to pray before his death due due to a severe illness that prevented him from doing so?

Prayer is not one of the acts of worship in which substitution is permissible. Therefore, it is not legislated for anyone to pray on behalf of another. However, supplicating for the deceased, giving charity on their behalf, and seeking forgiveness for them benefits them. Prayer does not cease to be an obligation for a Muslim adult as long as they are sane, and if they are ill, they should pray according to their condition.