The company I'm trying to help out has their mail server(groupwise)
sitting inside their firewall. They have a second netware file server
running internal(private non-registered domain) dns/dhcp. DNS on this
box points forwards unknown requests out to the firewall. All is
working just fine inside the office.

But, now they just hired a new person that travels fairly frequently.
Spending about every other week outside the office.

The public dns is hosted by an ISP and the mx record points to the firewall.

I'm having them create a new dns entry called
pointing to the firewall so he can see the mail server when on the road.

How can I have that same dns entry point to their mail server when this
user is inside the office?