A corporate First Officer reported that the Captain continued on a Visual Approach after losing sight of the non towered airport and descended below the MSA before getting the airport in sight again.

Narrative

Repositioning the plane after dropping off passengers the Captain and pilot flying determined that the weather at the destination airport was clear based off an earlier weather report from 20 minutes prior. Pilot flying had me cancel our IFR flight plan to do a Visual Approach instead of keeping the flight plan to the ground and getting an actual approach. After canceling the flight plan the pilot flying lost visual contact with the airport. We had the instrument approach loaded into the FMS but the pilot flying opted to keep doing the visual approach instead of getting back on an IFR flight plan. Eventually the pilot flying got us on the final approach segment while still on a VFR flight plan. At this point in time we were 700' below the MSA or about 900' AGL for that segment of the approach about 7 miles away from the airport. We were able to make a safe landing.The Captain has a tendency to fly 'freight dog' approaches and take altitude restrictions and visibility requirements as a suggestion. They also tend to cancel IFR even when we do not have the field in sight. Partially to reduce workload of talking to CTAF and ATC at the same time; and also the lack of a frequency to receive and cancel IFR clearances at the airport we hangar the plane at. The way to get a clearance or cancel on the ground is via a phone number that often gets congested as it is the only one serving multiple airports.