Would the flyby in this video be illegal under 91.119(a)?
This depends how you look at it but there are definitely some FARs this guy is in breach of. First one that comes to mine is 91.119 on minimum altitudes, it's pretty straightforward (assuming this is an area other than congested area)

Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

He is pretty clearly within 500 ft of that person...
You can see that there is a case for sparsely populated areas but I would argue that if there is a backyard party going on this is not a sparsely populated area.
Then as mentioned there is the catch all in.13

§91.13   Careless or reckless operation.
(a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.

I'd say that's reckless but of course that's up for debate. Then again it's impossible to say where this happened so we can't know what type of airspace this is in and if there were possible issues there. I would go on to say that at an altitude that low I would turn on my landing light to increase awareness and enable me to see the ground better.
If you really want to push it you could even go as far as saying that maneuver is aerobatic flight since it's an abnormal attitude (at the altitude it's being performed) and most likely not seen in normal flight. For the record that attitude looks to be about around 30 degrees which would be a standard steep turn (something I would never attempt that close to the ground).

For the purposes of this section, aerobatic flight means an intentional maneuver involving an abrupt change in an aircraft's attitude, an abnormal attitude, or abnormal acceleration, not necessary for normal flight.

If you think it fits under this definition the pilot is also in breach of 91.303

No person may operate an aircraft in aerobatic flight—
(b) Over an open air assembly of persons;
(e) Below an altitude of 1,500 feet above the surface;

I would consider a backyard party (as described in the video description) as an open air assembly of persons and he is clearly below 1500 ft.
The Counter Point: While the above points make a pretty solid case for the FAR's broken in the video, in theory if the maneuver is pre agreed upon, planned out, and executed as planned it may not be considered reckless. In the end of the day as a pilot you live at the mercy of the FAA and their decisions. In this case I would be most worried about that guy running into the propellor. I'm sure things like this happen more often than not.