A wideband is a must for 'e-tuning'. All an 'e-tune' really is a simple fuel cleanup and therefore relies on a wideband o2 sensor to monitor the afr's and therefore makes changes.

Do you need a wideband? Nope, it's not needed. Your friend got away with the stock sensors because the tuner most likely used the dyno's wideband and left it in open loop. The AEM ecu does not support the factory narrowband sensor (Haltech does though). In order to gain closed loop o2 control just like the factory ecu you will need a wideband.