A word/phrase for an unexpected change or turn of events in a person's life

Solution 1:

The term watershed is often used for this. From Cambridge Dictionaries:

watershed noun (BIG CHANGE)

​[Uncountable] an event or period that is important because it represents a big change and the start of new developments:
a watershed event/moment
The discovery of penicillin was a watershed in the history of medicine.

The origin of its figurative usage appears to be from the geological term, where a ridge or other geological feature separates flowing water into different drainage systems. Thus the metaphor is very similar to a crossroads or fork in the road, but perhaps with less implication of a choice of path (the water doesn't choose which way to go, just like "history" didn't choose in the example above).

So for your example

After that happened George's life would never be the same; he'd reached a watershed.

would especially make sense if the event "that happened" was not of George's choosing or possibly if George did not foresee the consequences. You could also say that

[The thing that happened] was a watershed for George; afterward, his life would never be the same.

or, even more simply,

[The thing that happened] was a watershed in George's life.

Note that a watershed can be good or bad, both in the causal event and in consequences.

Solution 2:

One common way is

Turning point

a point at which a significant change occurs

• Winning that game was the turning point of the team's season.
  That job was a major turning point in her career. (Merriam-Webster)

But as this phrase is used regularly, perhaps it's not dramatic or impactful enough for your needs.

One notch up, then, is an

Inflection point

1. Mathematics a point of a curve at which a change in the direction of curvature occurs.
2. US (in business) a time of significant change in a situation; a turning point. (Google definition)

And again from Investopedia (sorry for quoting the entire thing, but I think all the points are helpful and material):

An inflection point is an event that results in a significant change in the progress of a company, industry, sector, economy or geopolitical situation and can be considered a turning point after which a dramatic change, with either positive or negative results, is expected to result.

Companies, industries, sectors and economies are dynamic and constantly evolving. Inflection points are more significant than the small day-to-day progress typically made, and the effects of the change are often well known and widespread.

Based on mathematical charting models, the inflection point is where the direction of a curve changes in response to an event. In order to qualify, the shift must be noticeable or decisive and attributed to a particular cause. This principle can be applied to a variety of economic, business and financial information, such as shifts in the gross domestic product (GDP) or changes in security prices, but it is not used in reference to normal market fluctuations that are not the result of an event.

Solution 3:

The words "epiphany," "revelation," "realization" and "turnabout" all come to mind to describe a shift in perception, however these mostly have a positive tone and I'm not sure if that is what you have in mind.

Solution 4:

In addition to Dan's answer I would also suggest: pivot point, tipping point, changed gears, fast tracked (all these phrases used with 'unexpectedly' could create the mood that you are going for)

Solution 5:

I would suggest "crossroads." It suggests multiple possible alternate paths, and also implies (somewhat) not turning back.

"a point at which a crucial decision must be made that will have far-reaching consequences."

In the OP's example:

After that happened, George's life would never be the same; he'd reached a crossroads

Another alternative expression might be "point of no return:"

After that happened, George's life would never be the same; he'd reached a point of no return.