Stander-by

Something like the following conversation took place between me and Ben during The Color Run.

Me: Hey, let's ask that stander-by to take a photo for us!
Ben: Ha, "stander-by"!
Me: What? He's not passing by, so he's a stander-by!
Ben: I would have said he was a bystander.
Me: *face palm*

I knew it didn't sound quite right, but I couldn't work it out...