Your friend has spoken the truth with regards to your trans fluid - a complete fluid change might do more damage than good at this point, but your tranny really should have fresh fluid. I would start by changing a small amount at a time - drain out a quart or so, then top it back up and put several hundred miles on it. If you don't develop any problems, do the same thing again. After 4 or 5 times your fluid should be somewhat less brown and you should be able to tell if you're going to have any problems with it or not, and you can do a complete fluid change. Use the search function - there's a great write-up on here about how to do a thorough fluid flush.