No problem, I've done it plenty of times.

In fact, my most recent experience doing this, the other passenger was free to choose any Preferred/MCE seats (except for the ones reserved for full fare/elite customers) for free as if our itineraries were still connected, even though we were no longer flying together. (This is by no means guaranteed, though)

No problem for the ones in the original party. They simply create a new PNR for the person that was split off, the rest stay the same.