No it will work but just not wirelesly. Email will be redirected wirelesly as long as his workstation is turned on and he is logged onto it. Workstation can be locked though. If he reads an email message or reply and forward they will not be marked as such in Outlook unless he connect his berry with wire and sync. Yes, you can. It isn't supported but will work. And yes, unless any of these are not running all the time nothing will work.
Make sure you uninstall first BES before installation of BES on XP.
Out of curiosity their network is SBS2003 or something like that, isn't it?