The man told court he was at a friends' home preparing for a wedding when the fire erupted.
He said he and his friends rushed to the scene, and tried to help.
The fire caused severe burns to a then-17-year-old.
27-year-old Sean Nepinak and 25-year-old Seamus Nepinak are facing four counts of arson and one count of participating in the activities of a criminal organization. Both have pleaded not guilty.