The Payware he has bought through Steam will always stay in his account and can be downloaded, but why is he re-installing? A Steam "Verify cache" is the usual way to go, unless he has deleted all his Steam and Railworks files.
Non "Steam" Add-ons will have to be added from his Back-ups or re-downloaded from the supplier (if this is possible) if no Back up's were made.
If he is trying to re-install due to an error which was caused by one of his non-Default files and if these have not been deleted from the Railworks Folder, the error will re-occur.
Come back with more details if this is the case.