...and if they only need to send then set the default filter to not forward mail.
You can also set up BIS via a web browser on your PC. BlackberryFAQ.com (via Blackberry.com's Blackberry Smartphone Setup pages) indicates that Bell Canada's BIS site is at:
https://bis.na.blackberry.com/html?brand=bell
They'll need to modify the Send Using field when they want to send via the Helpdesk email address.