Short answer, "It depends."
Long answer, It depends on the officer, his department policy, whether he's carrying his weapon (he should be, typically), his badge (again, should), and handcuffs, the severity of the crime, danger to the public, etc.
A lot of it comes down to the officer's own judgement: It is better for him to get involved, out of uniform, or better for him to call for uniformed officers to arrive. Also, is he by himself or with his family, what is he doing, etc. An officer with his family is generally going to be less likely to get involved.
Note, there is a lot of "should," "generally," etc., here. Like I said, "It depends."
What do you need to have happen to work in your story? Figure that out and you can tailor it so the officer does or does not act and have it be believable either way by changing small details of the event.