A Catholic priest would not ask an Orthodox believer to repent of being a heretic and would be permitted to absolve him of his sins. There would be the presumption that, although the believer may have been objectively in error in some of his beliefs, he held them in good faith and not in rebellion against God or the Church. Therefore, subjectively, no sin would be imputed to him.
If I were in danger of death and was asked by an Orthodox priest to repent of heresy (which i don’t know that he would ask) I would have to reply that I was not conscious of being in heresy but, if indeed I was, I most earnestly beg the Lord to have mercy on me.
Catholic sacraments are available to Orthodox believers in case of true need. According to the response to my question, Orthodox sacraments are available to Catholic believers who are in danger of death. I would certainly ask for them. They are the same sacraments although we may define them differently. Orthodox priests are true priests and their sacraments are true sacraments and connect me with Jesus. If I were in a location where I were unable to go to mass, I would attend divine liturgy and praise and adore the Lord even though I could not receive him. I accept the Orthodox patriarchs have apostolic succession and I would be bound by their discipline as to who can receive the sacraments.
Being Catholic and believing that the pope is the successor to Peter and has a kind of primacy that the Orthodox don’t accept, I would be in heresy if I formally joined the Orthodox church. But I wish it were possible for us to be united.