the problem i see is not that the tank is overstocked. it probably is, but that is currently the least of your worries.
the main problem is the lack of water changes. the fish in the tank already have slowly become acclimated to this "thick" water stuffed fuill of the byproducts of their own waste. they have also lost their ability to adapt to clean water. a new fish coming from clean water would not be able to adapt quickly enough to handle this filthy water, and would go into osmotic shock and die.
likewise, if you were to do a large water change in your tank, your fish are so used to dirty water, and have lost their ability to adapt, they would also go into osmotic shock and die.
the only solution would be to start doing very, very small water changes. maybe 3-5 gallons at a time every day to start cleaning up the water very slowly. after a few weeks, the water should be clean enough that you can start doing larger changes.
bettagurl, if your friends tank is already overstocked, why are you buying her more fish?