These are only suggestions as it sounds like you got it under control.
1. Switch the guy to T-Mobile completely. He can take his phone number with him. Then he could use one phone everywhere. That's what my company did. T-Mobile even installed in-building antennas in our building as we are in a dead phone for about every carrier(even VZW). I'll give you the name of our rep(super-guy) if you are interested in such a scenario.
2. Use the T-Mobile web client and forward everything while still showing his original email address. I assume you can still get a web client version since that's the cheapy version that the BB comes with. http://www.t-mobile.com/bis/default.asp