Asking for clarification...
You would have said no to the player asking to apply disadvantage to reflect her fear even though she could have chosen to not attack at all "to reflect her fear" or chosen to just ignore her fear and attack fully? Both extremes would be allowed but not the middle ground?
main reason i ask is not to criticze your table rules but because others seem to have considered a Gm choosing thusly to have been a strawman case, not one represented here in spite of comments leaning that way.
Thanks.
Hmmm... edit for additional query as well... if the player wanted to reflect the fear by choosing "i will only take one attack of my multi-attack" (assuming appropriate ability) would you permit that but deny the "take both at disadvantage" request?
As to your first question, I would have asked the player to clarify what they wanted to do and how they go about making that happen. Until I know both of those things, I don’t know what rules I ought to be applying. I might even need further clarification.
A player might say, “I feebly attack the enemy with my sword because of my deep fear.” Now ok that’s all well and good. But I need to know if the player is more interested in the attack or in portraying the fear, before I know if I want an attack roll, or perhaps a wisdom save, or maybe if I just want to narrate what happens next. But what I would not do is just let the player pick what rule resolves the situation.
I might skip a die roll and give a quick “your half-hearted, terrified assault is effortlessly knocked aside by the enemy as they bear down on you. What do you do?” In which case I’m giving them the portrayal of fear-via-feeble attack as a freebie and then giving them an action they can take “for realsies.”
Which sort of gets to your second question. I would absolutely let the player declare their own actions. That’s their job. And I have no problem with a player making a sub-optimal choice. Happens all the time anyway - faced with a tough situation, players don’t always make good choices. But I don’t tell them “No, you don’t pick that, you pick a better option.” They’ve got free reign to portray their character and decide its actions. This includes the freedom to be right, wrong, different, weird, normal, abnormal, or whatever. But they can’t adjudicate their own actions, they can’t decide to make an ability check. They can take an action that might require one, but they can’t just skip to “I roll a 17 strength check so X happens” just the same as they can’t say “I attack with disadvantage.”
I mean heck, what if, unbeknownst to the player, they have a secret advantage? If I tell the player they can’t roll with disadvantage, or to roll only 1d20 (bc the secret advantage cancels the disadvantage), what then?
Corner case for sure. But to the point of roles. DM sets up a scenario/challenge, Players make decisions to change to scenario/overcome the challenge, DMs adjudicate those decisions.
(It’s perfectly fine by me if a group doesn’t have the same roles I do. But I find I get like zero rules lawyer arguments this way, versus when I let players pick what rules they want to apply. And since that means more play time at the table, that’s how we roll).