"American Idol" alum Haley Reinhart's friend was taken down by bouncers and choked so hard ... his entire head was blood red during the bar brawl that led to Haley's arrest.

TMZ obtained video of the melee, and it's intense -- 2 bouncers at the Lamplighter Inn, outside Chicago, had Haley's male friend surrounded and at least one of them had his hands wrapped around the friend's neck.

Haley was right in the middle of the fracas and you see her square off with one of the bouncers, who accuses her of punching him in the head before tossing her out of the bar.

The brutal video seems to support what Haley's attorney told us -- that bar staffers had assaulted one of Haley's friends, leaving him with multiple injuries.

As we first reported, Haley was booked for misdemeanor battery.