Aux and Microphone to same input
An 'aux' input is aka a 'line level input', around 1Vpp ac.  A microphone input can be one of several configurations to support various types of mic.  The two are not compatible.  And you roommate knows enough to be dangerous - analog audio is an AC signal, goes +ve &amp; -ve, so putting a diode in series will at best kill half of your audio &amp; sound horrible, but as he said, it has a 0.7V forward voltage drop, which is enough to kill most of a 'aux' signal anyway - result will be silence or near-silence (&amp; that's before getting anywhere near the amp's input impedance &amp; whether the diode would actually turn on).  In other words, buy him a beer and tell him to stick to what he does best :).

What you probably need is some 3rd-party 'line-level + microphone pre-amp switch' to achieve the combined functionality you're looking for.