I'm assuming your dad remotes into his other computer at work, and you'd like him to be able to remote into that machine via the Internet. I'm also assuming your dad's second computer is in that same office and he's not using the Internet to access it, just the LAN.
Simplest thing to do is open up port 3389 on your dad's firewall at work, and forward incoming TCP traffic on that port to the internal IP address of your dad's other computer. However, this is bad for security (anyone can try to remote in and try to guess passwords, etc.) It's possible to set up the office router to allow this and do a bunch of other things, but I'm assuming your dad is sharing the office with others and won't have the authority to change network or firewall configuration at will without going through someone else.
Probably the easiest and best thing to do, unless your dad wants to deal with the office's IT department, is to sign up for something that uses an intermediary system. I believe either LogMeIn or PCAnywhere is going to do what you need.
Since your dad's second computer at the office likely can't accept arbitrary incoming connections from the Internet, what needs to happen is that your dad's second computer connects to an intermediary service. This works behind NAT and firewalls because that computer is initiating the connection, which is allowed. Then, when your dad wants to connect to that computer, he logs into the intermediary service, which then uses the existing connection your dad's computer started to actually initiate and setup the session.