He can probably direct connect to you if he can to other servers.
Alternatively ask him go to Steam -> Servers and add your IP to Favorites as a Favorit server. It will not appear in Steam but it will appear in game >> Multiplayer >> Client >> Favorites.
In case your provider puts you behind a router your server will not be visible in Client section but you can start server in Lobby section to make it visible from behind a router.
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