Site hosting with ISP, email at another – is it possible

I’ve set up a wordpress based website for a friend which is hosted with one ISP but he wants to retain email and DNS control at his existing ISP – currently there’s an HTML redivert to the temporary URL the site uses.

I’ve been told that it’s possible to do this by changing the host header in the httpd.conf file. Would be very grateful if someone could let me know whether this is the case and, if so, what, more specifically, needs to be changed.