AirTran no longer has interline agreements with other carriers. They'd have to write a check to put him on US Airways, which isn't going to happen for a weather delay.

What I would ask for in this situation is to buy a PHL-CLT ticket on US Airways and request a refund for the PHL-ATL-CLT AirTran segments. Of course the US Airways ticket is going to cost a pretty penny since it's a fortress hub to fortress hub route, but if you've got to get there it might be worth the cost vs. spending the night in ATL. Technically AirTran tickets are non-refundable (except for full-fare business class) but I've found many agents to be flexible and allow refunds during irregular ops such as this.