Insurance? the same as buying a new motor car. don't move it onto the street until you have a cover note.
from your side, you should see a letter to him from his broker (who has attained same from the actual insurance company) mentioning your name as pilot and the type of flying / operation that it authorises you for.
if any / all conditions are acceptable and can be met, away you go.
otherwise his insurance company might say, who's he? whilst waggling the accusing finger. so might your 'friend??'