The flame arrows spell enchants a quiver so that every arrow/bolt fired from that quiver deals an extra 1d6 fire damage. I assumed that any creature (including the caster's allies) can draw and fire these arrows.
The caster has the Flames of Phlegethos tiefling racial feat (Xanathar's Guide to Everything, p. 74-75), which has the following benefit, among others:
When you roll fire damage for a spell you cast, you can reroll any roll of 1 on the fire damage dice, but you must use the new roll, even if it is another 1.
It is obvious that if the caster attacks using the flame arrow (an arrow from the enchanted quiver), Flames of Phlegethos would work for them because they are the one cast the spell. However, how about the other creature firing the flame arrow? If they are not the one who cast the spell, can they reroll the extra fire damage on a 1?
Best Answer
Yes, a 1 on the damage could be rerolled
Reading the rule literally, the feat only applies if the spellcaster is the one physically rolling the damage dice. So the question is whether or not the caster can roll the dice for their Flame Arrows spell.
The spell text says:
Fireball, which I think most tables would agree is unambiguously covered by Flames of Phlegethos, uses similar wording.
It seems reasonable to me to read that the Flame Arrows spell is applying the damage, and the caster of the spell is entitled to roll the d6 (and reroll as appropriate) regardless of who is firing the bow.
At my table, I would probably let the archer roll the fire damage for expediency, but the rerolls would still apply.