Does the Hideous Laughter spell make a flying foe prone and cause the foe to fall?

Question

A witch that's using the flight hex to fly 20 ft. up in the air fails her saving throw against my halfling bard's hideous laughter spell.

The hideous laughter spell description says that an affected creature

collapses into gales of manic laughter, falling prone. The subject can take no actions while laughing, but is not considered helpless.

Does the witch become prone in the air?

Would the phrase take no action effectively mean that the witch falls to her death, grappled and subsequently slain by her own foolishly placed black tentacles?

Answer 1

The creature falls down before becoming prone.

A flying creature cannot be rendered prone. However, the FAQ on flight states:

Flight and Magical Flight: Can a paralyzed or stunned creature keep flying with magical flight? Does a creature with magical flight not apply bonuses or penalties to Fly checks because it doesn’t have a “natural” fly speed? Does flying make a creature immune to being flat-footed?

No, any creature that loses all actions can’t take an action to attempt a Fly check to hover in place and thus automatically falls. That includes a paralyzed, stunned, or dazed creature. Magical flight doesn’t act any differently, even for paralysis, as it isn’t a purely mental action. A creature with 0 Dexterity can’t fly, and paralysis sets a creature’s Dexterity to 0. Despite the fact that the Fly skill mentions that bonuses and penalties from maneuverability apply to creatures with natural fly speeds, they apply for any fly speed. If they didn’t apply to creatures that gained flight artificially or through magic, then those maneuverabilities (like the listed good maneuverability for the fly spell) would have no game effect. Finally, the statement “You are not considered flat-footed while flying” means that flying (unlike balancing using Acrobatics or climbing) doesn’t automatically make you flat-footed or force you to lose your Dexterity bonus to AC; it doesn’t mean that flying makes you immune to being caught flat-footed.

Since the creature cannot take any actions it thus cannot hover and falls down as per the normal falling damage rules. This means that if they hit the ground after falling 10 or more feet, they take the appropriate falling damage and are rendered prone automatically. If they fall less than that, they are rendered prone from the spell.

Answer 2

Flying creatures can't be rendered prone

Pathfinder creative director James Jacobs addresses whether or not a flying creature can be rendered prone in this 2012 exchange:

Can a creature… with natural or magical flight be knocked prone?

Not when they're flying. You can mess with a creature in the ways detailed in the Fly skill, which might make them fall, but you can't make a flying creature prone. Or a swimming or burrowing or climbing creature, for that matter.

Absent this pronouncement, the GM determines the effect of rendering a flying a creature prone. Note that flying creatures were already immune to the trip combat maneuver (which, when successful, often results in a prone foe): "Some creatures—such as oozes, creatures without legs, and flying creatures—cannot be tripped."

Creatures that fly usually fall if they can't take actions

As per the FAQ and as cited in this fine answer to this earlier question and this fine answer, "any creature that loses all actions can’t take an action to attempt a Fly check to hover in place and thus automatically falls." This includes a creature that's flying without the use of wings (like a witch employing the hex flight to use an effect like the spell fly). The spell hideous laughter makes it so the subject can "take no actions" for the spell's duration, so a flying subject usually falls.