Chris, you've opened up quite a can of worms here. What do you want to do with the machine? Prototype work? Production work? A combination of both?
From the sounds of it, your friend has what is known as a hobby type machine. Nothing wrong with that, but don't expect to do anything but simple stuff, and it will be very, very, slow.
You need to know if the machine can read G-code, the language of the CNC machine. If it can, then software like Surfcam or Mastercam will work well (as Matt pointed out), but be advised that it will be expensive. Eg: I'll be upgrading my seat of Mastercam this year to the tune of ~$15K or so.
There are, however, other options, but expect to spend at least a grand.
Some sites to check out:
http://www.cnczone.com/ (a mix of hobbyists and professionals, but slanted toward the hobbyist)
http://bbs.homeshopmachinist.net/ (obviously geared toward the home shop guys)
http://www.practicalmachinist.com/ (the Pelican Parts of the CNC world. Mostly professionals, and some great info can be found if you search)
PM me if you'd like more info. I've been in this game for close to 20 years, and still don't know what I'm talking about
