If Emacs would only get a relative file name it would have no chance
to find the file. As I wrote in another letter it also gets the cwd,
but in those cases we are discussing here it gets the wrong cwd.
So this is actually a bug that we should fix. It should be rather easy
to do that by just using the cwd for the given file name's drive so to
say.
There can be several file names with different drives on the command
line, and I don't know whether Emacsclient knows which command line
arguments are actually file names.
If there is a good solution that is not excessively awkward, it might
nice producing it, but I am not sure of that.