I recommend he look for a place in his area that rebuilds automotive starters. They should be able to rebuild his starter unless it needs parts that aren’t available. I ran a quick web search and came up with the listing below in a suburb of Detroit. If they can’t do it, they may know someone who can.
-Note the word “Marine” is on the outside of the building.
May be worth giving them a phone call. https://www.yelp.com/biz/bills-suburban-alternator-and-starter-specialist-roseville-2