it seem to be a issue related to a slow connection or something blocking him from connecting (router ports etc?) i cant say im familiar with rust and its networking but i have had the same issues while my internett have been slow etc. i suggest as a starter to allov rust thru the windows firewall and if you know the rust networking ports unblock these from your router.
He has no other issues with other online games CS, Dota, WoW etc..
I've been looking for the rust networking ports to forward but i haven't come across an answer on that one yet; which is why i setup DMZ on the router/modem for my LAN IP and not even that worked.