I would call it a bug if it were happening for you and me too, but it's only happening to him. Has he tried different exe's in different directories with the same results? What about $msfile .. does that behave the same way for him?
//echo -a $msfile(*.*,$+(Choose the executable file of your media player)) $msfile(1) <-- this is msfile
1 C:\Program Files\Winamp\winamp.exe <-- this is msfile
~ Edit ~
For qwerty
Thanks to qwerty for my education in evaluation brackets in this thread, I know this...
//echo -a $msfile(*.*,$+(Choose the executable file of your media player)) $msfile(1) <-- this is msfile
1 C:\Program Files\Winamp\winamp.exe <-- this is msfile
can be replaced using this...
//echo -a $msfile( [ $msfile(*.*,Choose the executable file of your media player) ] ) <-- this is msfile
C:\Program Files\Winamp\winamp.exe <-- this is msfile