Blues knows this but for the benefit of the others, handcuffed does not automatically equal effectively subdued.
That out of the way, in your situation it is your duty to stop the abuse of a subdued and restrained suspect. A suspect in your custody is now your responsibility. In addition to your duty to protect your suspect now that they are subdued and restrained, you may have a duty to protect your fellow LEO from themselves. Having worked child sex crimes cases I can tell you what we used to call “hate discipline” is a thing.
Regarding the scenario in the article. In the United States, in the early 21st century, the need to intervene in a scenario like that described in the OP is extremely rare.
There is a reason most states and the federal government make resisting arrest and obstructing official duties criminal offenses. If the LEO does you wrong address it in court, not the street. They do this because for every scenario like the one in the OP, you get a hundred thousand assholes who will resist arrest and claim it was self defense. Same for the family members and “looking lou’s” on the street trying to jump into what they “feel” is an unjust arrest. There is a place to address those concerns. It’s called court.
Same thing with enhanced charges for assaulting LEOs, firefighters etc. because without them you have open season on people who have to do a job that is already difficult and dangerous. If you haven’t been in a fight since elementary school or you have never been knocked on your ass and wondered if you are going home today you are probably out of your lane on this.
Like a lot of libertarian ideas it briefs well but doesn’t work out in practice. In fact such an “open season” and “free for all” would result in increases in both the incidence and severity of LE use of force.