I have watched a few games where the Tasha's hideous laughter spell has been used before and the DMs there didn't count the creature completely out of the fight. For example: A player next to affected creature ran away after casting successfully, DM gave the Bandit an attack of opportunity with disadvantage due to the spell saying the creature goes prone.
We basically had the same scenario and I ran with that idea but the bard argued that the target was incapacitated as in the description.
What do the rules say is the correct way to handle this?
TLDR: If a creature is prone, e.g. due to Tasha's hideous laughter, can it still make opportunity attacks if an adjacent foe moves away from it?
Best Answer
Incapacitated will prevent opportunity attacks
Your player is correct. The opportunity attack requires a Reaction. And Tasha's Hideous Laughter's description states (PHB, p. 280, bold added):
And the "incapacitated" status states (PHB, p. 290)
So if someone is under the effect of Tasha's Hideous Laughter, they can't take the Reaction necessary to make an opportunity attack. They'd be able to crawl and do things that don't require actions or reactions (or bonus actions) such as speak, but can't do much else. They'll get a chance to break this spell with a Wisdom Saving Throw at the end of their turn, and any time they take damage.
NOTE: The "prone" condition doesn't prevent a character from making opportunity attacks (they will just make them with disadvantage). It's the "incapacitated" part that stops them.