Hmm... on thinking on it further, I think neither my friend nor I were correct.
First, I want to say I still disagree with "the target" meaning "all targets of the at-will". It clearly says "the target" - as in the singular.
As well, its not a power that triggers off the use of an at-will - rather, its a power that lets you use an at-will on the target. Whether or not that at-will might have other targets is IMHO irrelevant to the target of THIS power.
That said, it DOESN'T say that the extra damage is only to the target - but rather that it only triggers if you hit the target. So, if you are using an at-will that affects multiple targets and only rolls for damage once (such as most/all bursts/blasts), then if you hit the target of Learned Boost, you do the extra damage to ALL the targets of the at-will. Indeed, given that Learned Boost essentially says “add x damage if you hit the target”, he may get to add the extra damage even if the power did have multiple damage rolls… though I don’t think I need to decide on that just yet, as I don’t think my player has such an at-will. have such an at-will.
Or, if you miss the target of Learned Boost, you don't get the extra damage to the other targets of the at-will, but you get to keep the power (as it is reliable). This happens even if you hit other targets of the at-will... given my comments above about the meaning of the word "the target" in Learned Boost.
And, in my view, such an interpretation wouldn't put it out of line for the damage one would expect from a lvl. 11 paragon path encounter attack power.
But, yeah, I agree that its terribly worded.