Odd connection problem (proxy + internal)

Hope someone has some ideas on this one because I'm fresh out! :)

I have a SQL server on our internal network behind a linux firewall and
Proxy server 2.0 (WinNT 4.0). It is the backend for an accounting
system and can not be placed facing the internet (even in the DMZ). I
have several users connecting to it from their workstations in the same
subnet. We have a sister company that has an accounting manager here
that belongs to our network and also connects to this accounting
package. Now the tough part, he has an assistant here that doesn't
belong to our network but rather VPN's via a wireless internet
connection to the sister company to access systems there. Now he wants
this assistant to be able to access the accounting package (via file
DSN's) to our internal SQL server.

I have read KB216415 where it shows how to access SQL through Proxy 2.0
by installing the Winsock proxy on the SQL server and mapping the ip
address to the external IP of the proxy machine. But then it states
that the SQL instance will no longer be listening on the (internal) IP
address but rather the winsock proxy as shown here:

2002-02-12 17:11:32.28 server SQL server listening on Winsock Proxy,
Shared Memory, Named Pipes.

I'm guessing here.... but it seems like this would break the current
connections and I really don't want to do that.

The assistant can not join our network and VPN out through the proxy
machine as 2.0 doesn't support it. Besides, their network has had
problems with malware and I'd rather not have any kind of direct
connection between mine and theirs. I also thought about putting
another nic in the assistant's machine and a firewall between that card
and our network to limit access but this seems to be overkill.

Ideally, SQL listening on the current IP address and at the same time
listening on the winsock proxy would solve the problem but I've not
read anything that mentions this as a possibility.

Any ideas would be appreciated much.

Thanks,
Ken