The Blackberry technology is largely based around the idea of "push-email", which means that emails come to you almost instantly, rather than you physically have to check your email. I use a Blackberry 7230 and find the email service to be excellent. Basically I have my Gmail account forward all incoming mail to my provider-supplied Blackberry email address, which means that I literally get them within seconds of them being sent. I also have it check my university POP3 account -- POP3 accounts are polled every 15 minutes, so there may be a little delay, that's why I prefer forwarding. Although different BBs exist, they are all aimed primarily at fulfilling this function.
The Treo, however, is more of a regular PDA/phone. It will, of course, fulfill the role of checking email, being a phone, like the BB, but will lack the push functionality (which, must be noted, does usually incur an additional monthly charge). I suppose it depends how much your other-half will be using her device as a phone vs. a pda vs. an email centre. I have a reasonable amount of experience with such devices (Sony Ericsson P900, now P910i, O2 XDA, now XDA II, Blackberry 7230 & I work in a mobile phone shop) and in general what I find is that there does not yet exist a device which really does well all of what you want. Devices which are great as phones (I find the SE Pxx0 range pretty good here) are never quite such great PDAs, good PDAs (like the XDA range) aren't great phones, and I find Blackberries pretty niche, in that they're great for email, but don't make brilliant phones or PDAs. FYI, I usually carry my SE P910i & my Blackberry as I feel they're both necessary, as neither fully does the job of the other.
Any more questions, don't hesitate.
