How does the ring of mystic fire interact with scorching ray?

In my D&D 3.5 campaign, one PC just got a ring of mystic fire, and I’m trying to figure out how the ring works with the spell scorching ray.

The Magical Item Compendium says

A ring of mystic fire provides a boost to your fire-based spells. When casting spells with the fire descriptor, you gain a +1 competence bonus to caster level. This is a continuous effect and requires no activation. The ring has 3 charges, renewed daily at dawn. Spending 1 or more charges grants a bonus to the amount of damage you deal with the next fire spell you cast before the end of your turn. (If the spell doesn’t normally deal fire damage, this expenditure has no effect).

  • 1 charge: +2d6 fire damage
  • 2 charges: +3d6 fire damage
  • 3 charges: +4d6 fire damage

My player intends to use all 3 charges for +4d6 fire damage with the scorching ray spell, which says

You blast your enemies with fiery rays. You may fire one ray, plus one additional ray for every four levels beyond 3rd (to a maximum of three rays at 11th level). Each ray requires a ranged touch attack to hit and deals 4d6 points of fire damage.

The rays may be fired at the same or different targets, but all bolts must be aimed at targets within 30 feet of each other and fired simultaneously.

My question is this: Does the ring of mystic fire grant the character a +4d6 fire damage per ray? I’m inclined to rule that the ring does add its damage to each ray, but that risks making the spell overpowered, even though it is only once per day. And, even if overpowered, I prefer to rule in favor of what makes sense.