I'm planning on shooting a documentary on a potentially important shift in a friend's life. In an attempt to educate myself a little bit before shooting the events, I'm reading about the classic structure in three acts, with act one depicting a normal life suddenly disturbed by an event (which includes a wake-up call, doubts from the main character, and a decision to tackle the conflict/problem).

This is all good for fiction, but in the case of a documentary, the decision to make said documentary may come specifically because the dramatic structure (in real life) is already approaching the end of act 1. 

It translates into something like: "Well, this man's life, which was quite normal and safe until now is probably about to change because of what happened to him two months ago. His decision to make a radical change in his life might lead to interesting stuff... let's document this and make a movie about it!"

How does a documentary filmmaker deal with this? Should he explain instead of show? Does the linear structure (normality leads to an unforeseen problem, leads to a wakeup call, leads to the doubts on whether the character should do something about it, leads to the firm decision to actually tackle the issue, leads to Act 2) of dramatic writing need to be thrown away in this case, or somehow recreated after the events?

In short: what are the "state of the art" solutions usually employed to tackle this problem (starting to shoot at the end of Act 1, while still conforming to a 3-act structure).