The G1000 is TSO146
https://buy.garmin.com/en-US/US/in-t.../prod9717.html
"dual TSO-c146a WAAS-GPS receivers"
He should just upgrade to an ADSB transponder. Say, $5k for the unit and another $5k for an Engineering Order and installations. Possibly an EO is not needed if the documentation already exists.