lostcauses-noregrets answered:

Oh dear, you have my sympathies Anon, that must be very tiresome. Tbh I’d be inclined to just give your friend a copy of the manga and tell them to read it!

The idea that Erwin and Levi’s relationship can be reduced to commander and subordinate is so wrong headed that it’s hard to know where to start. It’s certainly true that this is part of their relationship, but even then, the military chain of command doesn’t quite work as expected when it comes to Erwin and Levi. Erwin does give Levi direct orders throughout the manga, but there are several occasions where he asks Levi to do things rather than ordering him. I’ve already written a post about this here. The most notable example is in chapter 70 when Erwin asks Levi if he is willing to take the Titan serum and decide who to use it on. Levi actually calls Erwin out on this request, asking him why he doesn’t just give him a direct order.

Also if Levi had been following orders, later in chapter 84, there is no question that he would have given the serum to Erwin, as the person whose survival would benefit humanity the most. Instead Levi lets his personal feelings for Erwin get in the way, and he chooses to let Erwin die in peace, a decision that has ramifications for everyone. In later chapters, several characters including Hange, Armin and Flocke all make it quite clear that they believe Levi made the wrong decision. @tsuki-no-ura has written an excellent post explaining the implications of Levi prioritising his personal feelings which I can highly recommend reading.

If that isn’t enough to convince your friend, then you could always point them towards Isayama’s Answers book and Character Guide, or throw them at their head. Both books go into Erwin and Levi’s relationship in some depth, making it crystal clear that theirs is an equal relationship founded on mutual trust and respect.

And last but not least, Levi only calls Erwin “Commander” once in the entire duration of the manga, when they are reunited after the Uprising. That little pause … speaks volumes.