Shameful! But if his "penitents" did not know, or had any reason to suspect he was a fake, and if they confessed in good faith, then God's grace covers the sins. If I knew I had been to this charlatan, however, I'd most certainly go quickly as possible and make a general confession, especially if it were so long ago I couldn't recall the specific sins. I'm sure many traditional priests have encountered penitents in similar positions.
This is why it's recommended having a regular confessor, but most of us no longer have that luxury.