Oracle Database - How does one connect to an Oracle DB while using 11g on a lan?

My colleague has 11g on Windows 7; seems to have his TNS file in the right place...he doesn't know how to connect and frankly neither do I. Since he's always asking me about how to do things, I don't want to disappoint him. Plus I have my reputation to consider.

Is his Oracle instance configured to permit remote connectivity?

Is the port Oracle uses open in the company/intranet/pc firewall?  Can you ping the server?

What is the connection string you are using?  Replace the sensitive stuff with fake info prior to posting here.

Yeah, we can ping.  Turns out that there's an environment variable in Windows that directs Oracle to look somewhere other than the "default" folder for the TSN. 

We're waiting on a fresh install of Oracle per company policy and we'll try it again. If need be, will post the details.