I too have a Treo 300 with Sprint Vision.
Actually, I am quite pleased with the Vision service. For an extra $10 a month, I get unlimited Vision minutes to be used anytime.
She can use the Blazer browser that is built in to go to the website:
www.mail2web.com
...and then check her email that way. Has worked great for me.
The Blazer browser is pretty nice. I have used it to access my online banking and other sites..
I also use my Treo as a modem for my Powerbook with this software utility called Wireless Modem.
The only reason I am getting rid of my Treo is I have the urge for a Bluetooth phone to go along with my new Bluetooth Mac!
My only gripes with the Treo are the display brightness in daylight conditions, and the battery life.
Make sure she is smart about turning off the wireless mode when not in use.
Good luck!