The murky brown was most likely because at some stage, the previous owner had changed from the green to red, where the flushing didn't get rid of all the old coolant. It also could have been caused by simply topping up with the wrong one as well. If you mix red with green, it becomes brownish, depending on the ratios, and at best, the anti-corrosion properties of the coolant are reduced considerably, at worst, you get a brown gel forming in the coolant which can cause problems with other components.
Your mate would be correct as well. There is plenty of evidence around on the web on galvanic corrosion of cooling systems in bi-metal situations, where the coolant turns acidic and becomes the electrolyte. Cathode is usually the biggest iron part, and the annode the thinnest bit of aluminium. Best thing to do to avoid it: change your coolant every year, and don't try to swap from the green to the red, as flushing the system isn't as simple as putting a garden hose up and/or down the radiator hose, as you'll still get pockets of old coolant hanging around ready to mix with the new. If you use the same type of coolant, there is less risk in the coolant becoming less effective sooner.
I've got a water to air intercooler on my Landcruiser, and my mate who used to install them now doesn't, as every single one he's fitted has come back to him with problems of pinholes in the intercooler core. Mine hasn't been problematic, as I regularly change the coolant (the red type), although my next change will be to a specific intercooler fluid. More electrical devices seen to increase the likelyhood of galvanic corrosion as well, so the more electrical farkles you have, the risk increases.
I also do a yearly change of coolant, brake and clutch fluid. These fluids are relatively cheap compared to the potential repair costs, and whilst I could stretch out the brake and clutch fluid to every second year, it is still cheap insurance to do it yearly. Just another job to do when the tupperwear comes off
