If this form is done from code, this is quite easy, I would need access to the code, or just the file where the form is defined would be sufficient. I would quickly do the changes and send you back the updated version of the file.
If however the webform module was used, then the form is not defined in code, and access to the site is needed, with an account that has the right permissions to change the webform. Or, alternatively, we could do a Skype call and walk you through the process, which minimizes the security risks for you. If all works fine, 10 minutes would be enough for the call, but if we hit a snag, it may take a bit more.
Thanks, Călin