I don't think there is anything special you have to do to enable you web service to be called from SOAP.

I have written a few web services that have been consumed from VBScripts and from external (non .Net) apps.

When you say "he simply can't", what do you mean? Is he getting error messages (i.e., invalid XML) or is he just not calling it properly?

When you browse out to your web service page and look at a specific function, does it have both a SOAP and an HTTP post function?