Well you can use "anything" to edit the XML file ; its just that clever editors (like notepad++) make it easier to do it.
Since your client doesn't like notepad++, perhaps the more relevant question is what editors do they allow?
("anything" meaning any text editor ; worst case vi or the Windows normal notepad I guess)
I recall someone on these boards promoting their own "XML spear" tool, but if you can't run notepad++ I would think you would have the same problem with any other non-standard tool.
(I just use notepad++ though)