Skilled Companion gives one ally a bonus. They could have written it better.
Your confusion is 100% justified.
It is quite confusing that the target line says "one ally", and the effect says "any ally" rather than referring to your target. It's reasonable to wonder if that would actually mean this power affects every ally - because you're right, it would often mean that.
Normally, powers like this would say "your ally" (as in Fey Switch, PHB1 p134). The powers that don't refer to the target really do tend to pick someone entirely outside the target. There's several powers in the very same PHB this power is from that target a creature, then do something entirely separate with an ally who wasn't targeted at all: e.g. Commander's Strike, p145.
So who's it affect?
Going with the power exactly as written, you could interpret it one of two ways:
- You target one ally. Then you completely disregard this, and actually affect all your allies, by one interpretation of "any ally."
- You target one ally. You affect that one ally, as in the sense of "any one ally."
The second one makes more sense, but they should have written something like "your ally" or "that ally" instead. "Any" is a word that can be used in a singular or multiple sense, and this clarifies it to probably mean singular.
Thank goodness it's only a target/effect line. If this actually did something specific to that ally, in addition to its current effect line, I'd have no idea what the effect line would mean.
One check or multiple?
My DM seems to think that this power would only apply to one ally for a single check, but can't really explain why.
It applies to multiple checks. There's nothing limiting this to just one check. If it limited it to just one check, the first sentence would instead say something like this:
The next time your ally attempts an untrained skill check with a skill in which you are trained, they gain a power bonus to their check with a single skill of your choice equal to your Wisdom modifier.
Actually, this writing collapses if we poke it with a stick.
I'm going to take picking apart this power one step further: consider what the following really means.
Any ally within 10 squares of you who attempts an untrained check with a skill in which you are trained gains a power bonus to checks with a single skill of your choice equal to your Wisdom modifier.
By this exact writing: if you are trained in dungeoneering, and your ally is not, and they are targeted with this power then make a dungeoneering check, they can get a power bonus to stealth - whether trained in it or not.
It doesn't limit you to only picking the untrained skill. Also, it doesn't specify whether the power bonus lasts a while, or whether it only lasts for that roll (making a stealth bonus useless), because it doesn't say things like "a power bonus to that check" or "until the end of the encounter" which would make it clear.
Let's rewrite this power into what the author probably meant.
Range: 10 squares.
Target: One ally.
Effect: Choose a skill in which you are trained, but your ally is not. For the next 5 minutes, or until the end of the encounter, your ally has a power bonus to that skill equal to your Wisdom modifier.
Much more straightforward.
Tangentially: On targeting, and retargeting
The part that is confusing me a little is the one ally target. Is that simply saying that each time one of my allies gets the bonus I'm targeting them with a Ranged ability?
Even if this targeted everyone I'll clarify: no, you'd just target someone once, the first time you use the power. Everything in the power only happens when you first use it: it'd take something very different to make you target them over and over. For instance, the effect line would say:
The next time your ally attempts an untrained skill check with a skill in which you are trained, you may use this secondary action as a free action:
(a secondary action which targets one ally and gives them a bonus to the roll)