Thing with radiowaves is that they're tricky. The frequency tells you how well they can penetrate structures. The technology type is different than the frequency (and accordingly, the numeric readings on the phone will be different).
It's possible your friend already had a good 900Mhz and a very weak (but usable) 1800Mhz signal. CA helps boost both weak and strong signals (and that's the benefit). With Zong, if they don't have a frequency broadcasting in your house, then you're out of luck. The switching between 2G and 4G is a byproduct of fluctuating signal strengths (that's just how radiowaves work).