There are so many "what if's" to this scenario it is scary. What are the legal constraints on citizens arrest in the jurisdiction? In this country it is only for offences punishable by 2 years or more prison time by day(5am-9pm) and for any offence punishable by prison by night.
(The remaining 8 hours of the day).

What fears for his own or others safety did the member of public have? As said in the report not long ago an escaped inmate from the nearby prison killed a guard in his escape.

Handcuffs in front are a potential weapon. Handcuffs behind the back not so much.

Our department recently enacted an official policy. If there is no threat to life it is better for an off duty officer to stand back, watch, if possible take some notes, and report instead of intervening. It came after several off duty incidents usually where alcohol had been consumed that resulted in public embarrassment to the department (off duty cop who had been drinking pulled a cyclist running a red light during a pedestrian phase off his bike causing a fracture. Another off duty cop in a sole charge station had been at the 19th hole of the golf club for several hours on a day off when told of a vehicle crash on a notorious stretch of highway. Because he carried first responders kit, including oxygen, in his patrol car he attended and ended up being breath tested and found over alcohol limits etc).

But at the end of the day people will continue to be stupid and not think of the consequences.