No. I don't think you made the right call.
You say there is a barbarian PC with 'Winter Phoenix Rage', which has this effect:
You enter the rage of the winter phoenix. Until the rage ends, you gain regeneration 5 + your Constitution modifier. In addition, the first time you drop to 0 hit points or fewer, you can spend a healing surge as an immediate interrupt.
The barbarian also has a power or feature that lets them make an attack when they drop to 0 hit points. The specific power or feature is not identified, but I'll assume that it's worded like so:
The first time in an encounter that you drop to 0 hit points or fewer, you can make a melee basic attack as a free action.
The question I'm reading is, "After dropping to 0 hit points for the first time in an encounter, can the barbarian both spend the healing surge and make the attack?"
I can certainly see the logic behind your interpretation, but that's not how I see it.
It seems to me that both of these abilities have the same trigger, so if one triggers they both trigger, and therefore both will happen.
An argument could be made that the healing surge will prevent the character from reaching zero hp, and that this will somehow retroactively cancel the attack's trigger. If you see things this way, then the player can simply make the attack first, and heal afterward.
For the rest of the answer, rules are from the 'Action Types' section of the Rules Compendium, starting on page 194.
First off, the type of each action is important. One is an immediate interrupt, the other is a free action.
You can only take one immediate action per round, but you can take an unbounded number of free actions per turn, except in the case of free actions used to make attacks. You may only take one of these actions per turn. So, the PC can't spend the healing surge if they previously used an immediate action this round, and can't make the attack if they've previously used a free action to make an attack this turn. Additionally, you can only take immediate actions on other characters' turns, so the surge can't be spent on the PC's turn.
Assuming that's all fine, then we get to the trigger. The 'Triggered Actions' section says:
A triggered action is any action that can be taken only when a specific trigger occurs. A trigger is an action, an event, or an effect that allows the use of a triggered action.
So, the surge cannot be spent unless the trigger occurred, i.e. the barbarian must have dropped to zero hit points or fewer at some point. If the barbarian dropped to zero hit points or fewer, then the trigger for the attack has also occurred.
Now, the question is whether multiple triggered actions can be triggered by the same event. I can think of at least one question where this community has supported the use of multiple triggered powers on the same trigger, so I think that the answer is yes, although I cannot find a specific rules reference to cite.
We then need to consider when the actions will occur. Let's start with the surge. The 'Interrupts' paragraph of the 'Immediate Actions' section says that:
An immediate interrupt jumps in when its trigger occurs, taking place before the trigger finishes.
In regards to the attack, the 'Other Triggered Effects' section says:
If an effect has a trigger but is neither an immediate action nor an opportunity action, assume that it behave like an immediate reaction ... however, ignore this guideline when the effect has to interrupt its trigger to function.
As the 'attack' effect must interrupt the 'fall unconscious' trigger to function, we treat this attack like an immediate interrupt. Note that we are treating it like an immediate interrupt in terms of timing. This doesn't actually make the free action into an immediate action, so the limitation of one free action per round still does not apply to it.
An argument has been made that if the healing surge was used first, then it would prevent the character from actually reaching zero hp, and therefore prevent them from using their attack power. This could be an issue if multiple powers are triggered that could potentially invalidate their trigger, but isn't really a concern in this instance.
Here, the player can simply make their attack in response to falling to 0 hp, and then heal in response to falling to 0 hp.
Therefore, this is the sequence of events I would expect:
- The barbarian is damaged, dropping the barbarian's hp to zero.
- Both of the barbarian's abilities are triggered
- The barbarian makes a melee basic attack
- The barbarian uses a healing surge, raising their hp back above zero
- The barbarian remains conscious and standing
Note: There is some ambiguity on when the healing surge would actually occur. The trigger in this instance is falling to 0 hp, not being hit by an attack that would reduce you to 0 hp. So, are those separate events? This is important for determining the hp value after the surge, and seems worthy of a separate question.