Bandwith requirements are low.
On his end, he shouldn't need much more than your basic broadband connection. When I was in my old house I could client fairly fluidly on my 74kbps connection.
Usually it's not bandwidth you should be worried about, but RAM.
What's his limit?
Chunk loading is the bulk of data sent, and it measures about 46KBps in spikes. About 50% while walking, times 8, that comes about to about 184kpbs average on far draw distance if he does nothing but walking. Standing and mining drops it to less then 30kpbs.
I'm getting tired of all the lame answers to this question, so, I tested it myself. I took my iPhone, put a tethering app on it, hooked it directly to my computer and played minecraft 1.4.2 for 1 hour exact. I tracked the data/bandwidth usage and here are the results.
1 hour of Minecraft Play on Server: play.savagerealms.net
Sent: 1.4 MB
Received: 15.5 MB
I ran all over the place while in game, flew all over the place, teleported, spawned a bunch, played PVP a bit, created some shops etc... Did just about everything you need to do in a basic game session. So, there you have it. No more Lame answers.
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OldToby1 - Savage Realms Helper