In a 2-vs-1 fight, can a spellcaster cast Dragon's Breath on their ally while charmed by the enemy?

Question

5e rules designer Jeremy Crawford stated the following in a tweet about the dragon's breath spell:

Dragon's breath has two sets of targets: the creature you give the breath weapon to and the creatures in the area of effect created by the spell.

Imagine that there are 3 creatures in a fight: a wizard, the wizard's ally, and an enemy. The enemy is able to impose the charmed condition on the wizard. While charmed, can the wizard cast dragon's breath on their ally?

Since there are only three creatures, the charmed wizard should've realized that giving dragon's breath to their ally would enable them to use it against the charmer. It is akin to: "Hey, I can't beat this guy! Here, have a bat - go and beat him for me!"

Since being charmed forbids you from casting a harmful spell targeting the charmer, does it mean that being a potential secondary target for the dragon's breath spell prevents the charmed spellcaster from casting dragon's breath while charmed?

Can the charmed spellcaster cast dragon's breath in this situation?

Answer 1

The charmed condition permits casting dragon's breath

Casting dragon's breath on an ally with the intention to harm the charmer doesn't go against the relevant effect of the charmed condition:

A charmed creature can't attack the charmer or target the charmer with harmful abilities or magical effects.

The Twinned Spell metamagic option has different and stricter wording:

To be eligible, a spell must be incapable of targeting more than one creature at the spell's current level. For example, magic missile and scorching ray aren't eligible, but ray of frost and chromatic orb are.

Notice that the subject of the charmed condition is the charmed creature, whereas the subject of the Twinned Spell metamagic is the spell.
The charmed creature only targets the ally and nothing the ally does can change this fact. In other words, the ally's targets are not the charmed creature's targets, so the charmed condition doesn't care about what the ally does.
Meanwhile, the spell (dragon's breath) can target more than one creature over the course of its duration (the ally and whoever is caught in the breath) so it is not be eligible for Twinned Spell.

But consider additional effects

Many sources of the charmed condition apply additional effects to the charmed creature and these effects can prevent the charmed creature from casting dragon's breath on an ally with the intention to harm the charmer.

For example, the charm person spell says:

The charmed creature regards [the caster] as a friendly acquaintance.

And the vampire's Charm action says:

The charmed target regards the vampire as a trusted friend to be heeded and protected.

If the source of the charmed condition is like charm person or the vampire's Charm action, then the charmed creature generally would not give an ally dragon's breath, because the charmed creature would not want to indirectly harm a friendly acquaintance or a trusted friend.

Answer 2

Yes, the spellcaster can cast Dragon's Breath

Charmed is a simple and limited condition; its only relevant effect here is:

A charmed creature can’t attack the charmer or target the charmer with harmful abilities or magical effects.

Despite what happens later with the spell, the spellcaster doesn't target the charmer with a harmful magic effect by casting the spell, nor does he attack him. So the spell is allowed.

The charmer could try and convince the charmed not to do so with a social check (with advantage), but that's a more subjective roleplaying aspect.