If making her see the truth is more important for you than abiding by the laws and customs of war, dressing as the enemy is definitely a useful trick.

I think you're basically right - I'm just not sure that I do consider that more important for certain values of "the laws and customs of war". I've certainly been in arguments like this, and not least because I'm perhaps a prime example of someone "who sees all opposing arguments as soldiers to be killed" - something I'm trying to fix.