As WN flies only 737 aircraft, it is relatively easy for them to swap aircraft if a plane has a mechanical or other problem -- if the aircraft being used for a 2:00pm flight has a problem, they might substitute ("steal") the aircraft scheduled for a lightly booked 3:00pm flight and use it for the 2:00pm, and later substitute a different aircraft for the 3:00pm flight. Depending upon the nature of the problem on the original aircraft, if it's minor and they simply need to replace a part that takes 45 minutes labor, they may very well be able to repair the plane and have it back in service for the 3:00pm flight.

I had this happen to me a couple of months ago at LAX; the plane was closed up and ready for pushback when a problem was discovered. They ended up offloading us and sending us to another gate where they had "stolen" a plane which had been scheduled for a different destination.

As for your friend -- in irregular ops like this, in my experience WN will do everything possible to accommodate inconvenienced pax on other WN flighjts (they do not interline)... and if you are traveling someplace that involves a connection, they will look at reroute possibilities to get you to your destination. However, if you are only taking a single leg flight, you will typically be booked on the next available flight to your destination.