For the record, I don't agree that feather fall should be an immediate action spell.
I think feather fall was just fine as written in the PHB: a free action that you could explicitly take out of turn. The problem with feather fall as an immediate action (IMO) is that you can't cast it when flat-footed. Thus, if you're walking along the edge of a cliff and it crumbles, causing you to fall, you can cast feather fall, because you're not flat-footed (because there is no combat). But if you're walking along the same cliff and an invisible enemy pushes you over the cliff, you can't cast feather fall because you're in combat and flat-footed. I think that's dumb.
Yeah, that's dumb. I'd say, "at least uncanny dodge prevents you from being flatfooted, so there's that." But then just recently I was informed i had been using a houserule this whole time, because uncanny dodge
only prevents the AC loss from flat footed, not the condition itself *angry face*
Stupid RAW...
The write-up of immediate actions is also dumb. An immediate should ALWAYS burn next round's swift action, not have some funky extra clause for using it during your own action. Then you get pleasant questions like, "What if it's my turn and I already used my swift action?" Depending on how you reply, you then get "So...I can't use the immediate RIGHT NOW, but...if I wait for the instant my turn in the round is offically over, I can, even though it's still the same round?" or "Oh, you'll allow me to take it? So basically all the RAW is good for is to force you to use your swift BEFORE your immediate?"
I've always treated immediate actions as taking away next round's swift action, no matter when it's used, and it's yet to prove broken. In fact, partially nerfing the ever-popular Nerveskitter with that ruling has turned into a rather happy unintended consequence. (IME, most people would rather have their swift on the first full round of combat, rather than the surprise round if there is one)
EDIT: There are TONS of items and abilities that let you apply them after a confirmed attack, but before damage is rolled. Wall of Blades maneuver (ToB), Gauntlets of Giantfelling (MIC), Factotum's Cunning Insight (Dungeonscape)... Many more...
I'd let the player use the energy resistance after knowing he was hit but
NOT after learning the damage result. Of course, you may want to give him a second or ask him if he wants to use his devotion feat in the future, in case of an issue where he would have considered using it if given the chance. Can be as simple as rolling the dice and just delaying telling him the result, if you do die rolls "behind the screen."