What happens if the no show arrives after a standby has been given a boarding pass?

Most airlines have a gate cutoff time. For example for United it's 15 minutes (see https://www.united.com/web/en-US/content/travel/airport/process/default.aspx#boarding-gate) . After that time the airline can choose to deny boarding, even though you have a boarding pass and a confirmed seat.

Whether they do or not, depends on a variety of factors like check-in status, connections, status, mood of the day, etc. which are hard to predict and not particularly consistent.

You are never truly "safe" until you have pushed off the gate. You can certainly be de-planed after having boarded and I have also seen the doors being re-opened and passengers kicked out of their seats for late arriving passengers although I don't know if anyone was actually removed from the plane

Generally, the "no show" gets left behind and has forfeited his/her place on the flight. In that case, the no-show has no choice but to purchase a ticket for another flight.

You are safe as soon as you get the boarding pass. At least outside the US. I hear that in the US you can be dragged screaming and kicking out of your seat. But in the rest of the world, once you have your boarding pass and a seat assigned to you, you're good to go.

As for the person who was a no-show, or rather, in your scenario, a very late show, there are a few possibilities:

1. This person arrived after check-in closed. Next flight, brother.
2. This person arrived just before check-in closed. Depending on a few parameters, this person might be able to get a boarding pass, or, again, be left on the ground, crying and throwing a tantrum :-)
3. This person knew that it was too late, and went directly to the counters for the next flight, and was probably put on the waiting list.