Sometimes it's better to light a flamethrower than curse the darkness

Yeah it doesn't necessarily need a lot of intervention -- here, someone saying "hey the back of the line is over there" would've probably been sufficient, and it can be done in a minimally confrontational way, like "obviously I'm assuming we share a value of standing in line and I've charitably decided that all you need is to be treated like you've slipped up, as could happen to any of us!" The only message that really needs to be sent is "hey regardless of whether the person with that white cane or guide dog knows what you're doing, I know what you're doing."