Whether or not he can access his email externally in dependent on a number on things.

Let's start with, How does he access his email at work?

Is it in a client (Outlook, Thunderbird, Eudora) or Web-Based? If it is a client, does the client use POP3 or IMAP? Is he connecting to an Exchange server?

These all matter...

Next... just because the VPN is at the dyndns address (makes it sound like a small company, which makes me assume it's basically a linksys router on the perimeter) doesn't mean that email is available at that address. Some valid options.

1) The address isn't port forwarded on the router, and therefore isn't available externally and has to be accessed via VPN.
2) He uses a client at work and is trying to replicate that at home... but he has to access the web-based software while at home.
3) There's a server he can connect his client to, but it's at a different address then the VPN client.

Unfortunately there just aren't enough details to make a call on this.... In 4 companies, I've seen all three of these setups used...