There is still no clue as to if this is VB6 or .NET.

The complexity hinges on how you duplicate his accounting software.

If you code it correctly (i.e. separate the interface from the
rules and functions) then it will be less than one huge mess.

If you don't, you might as well consider writing two separate applications.
One for stand-alone and one for the web.
Or, just do it for the web and forget stand-alone.