I suppose you could bridge the two networks with a VPN - essentially make one network out of two, but you'd have to coordinate a lot of services to make that work.
The remote user could connect to some point outside his network, and then VPN into yours. This would make him appear just like one of your other field people.
If the remote user just needs to get to the SQL Server's desktop, there are other solutions (use VNC with DDNS and the appropriate configuration on your firewall) that will get him there, but I would worry about security.