Aura of the Guardian and Damage order of operations

The Oath of Redemption Paladin subclass gets the following feature at 7th level (XGtE, 39):

you can shield your allies from harm at the cost of your own health. When a creature within 10 feet of you takes damage, you can use your reaction to magically take that damage, instead of that creature taking it. This feature doesn’t transfer any other effects that might accompany the damage, and this damage can’t be reduced in any way.

It is clear that the Paladin cannot reduce the damage they take but it is unclear whether the damage the the initial target would have taken could be reduced. Is the rule that the damage cannot be reduced from its original damage roll or that it cannot be reduced from what the target would have taken?

For example:

Farla the fighter has the feat Heavy Armor Master which grants:

While you are wearing heavy armor, bludgeoning, piercing, and slashing damage that you take from nonmagical weapons is reduced by 3.

Farla is under the effect of Warding Bond which grants her resistance to all damage.

Farla is hit by a Storm Giant’s greatsword which rolls 30 damage.

Heavy Armor Master reduces this to 27 which is then reduced to 13 by resistance. Unfortunately Farla only has 10 hp remaining so Psi the Paladin uses Aura of the Guardian to take the damage instead.

How much damage does Psi take?