Will a friend be lost?

Imagine you have a good friend. That good friend has a part of them they haven't shared with you. They were unfaithful in a previous relationship but never disclosed it to you (they disclosed it to the ex who rightfully dumped them). They told you there were incompatibilities in the relationship as a cause for the relationship ending. What would you think and how would you feel if they told you? Would you keep them as a friend if it had happened a few years in the past and they had changed their ways? Would your answer to the last question be different if your friend's hesitation in telling you was because your own partner had worded staunch statement about there being no forgiveness, or redemption, even if the offender became a saint afterwards? Trying to understand what to do.