Why doesn't he just use the version of Caslon he started with? He must still have a copy of it embedded in the original document or on a disk somewhere no?
Because it's not supported by new versions of Windows or Word.
To ask the obvious, has he tried to convert the old fonts to OpenType? There are various free and proprietary tools to do that and Private Unicode Area ranges still exist.
To install a Type 1 font on Windows, you must have the .PFM file that normally accompanies the font's .PFB file and should be in the same directory as the .PFB file. You right click on the .PFM file and select Install.