The Qur'an is rather clear that everyone is responsible before Allah for his/her sins. While we may hope for intercession from the Prophet (S) or others, and it is fine to pray that other people be forgiven, it is not possible for one person to stand in for Allah and make the decision to forgive their sins.
We might sometimes get an idea that someone has been forgiven for something and reassure them, but it is not our place to issue a guarantee that this person has been forgiven.
(In cases where the Prophet and Imams told people they were forgiven, it is because they had knowledge from Allah that these people were forgiven, not that they personally engaged in the act of forgiveness on behalf of Allah.)
I would be rather skeptical about someone who claims to have this authority!