A token that creates a small bird that can be used to deliver a small
written message unerringly to a designated target. The token lasts as
long as it takes to carry the message.
Arguably if your target is a living person and that person no longer exists, it won't even try to deliver it. However, if they are in some prison cell it's a fat lot of good sending them, basically, a 'tweet'. Quite a literal tweet in this case. I mean what message would you send? The magic item is specifically an "unerring" delivery, so by strict rule definitions it will deliver it one way or another.
If you are willing to fork out 500gp, even a Level 3 Cleric can effectively get a spell of Level 7 Cleric. Though hiring a higher level Cleric's services to cast a Level 7 spell wouldn't even cost that much though that is all dependant on you finding a high level Cleric in time. It's a matter of which you find cheaper or easier.
This has permanent duration and no range limitation, if you had hit your allied with the benefit of this spell then whoever cast this spell would forever have the ability to know their direction and distance. So it's not so useful by itself unless your GM allows a soft-retcon or it "just so happens" that another NPC had done this to him for whatever reason.
Now, whether you can combine this with the Bird Feather Token... that is something else entirely. You know, hit the bird for 1hp damage (be careful) then try your best to track the bird till the "signal" disappears.
Slightly above your level so buying a scroll of this your party cleric (if you have one) could probably make the spellcraft check so you could at least get your the information you need from the Doppelgänger that you killed. They are probably going to know exactly what happened to the person they replicated just to make sure that they aren't seen side by side or whatever.
Another handy item that would have been useful if you'd known about it before but it's too late now. It's not really clear if you can be creative in trying to deliver items.
Good Ole Diplomacy
If your friend is alive yet not free then someone is holding them prisoner, someone is feeding them, someone is probably talking to them. When you get word out of a reward for information about their safe return, well someone is very likely to talk. Hell, even the most hard hearted jailers couldn't at least miss the opportunity for some blackmail and extortion, I mean if their doppelganger master is dead they might as well get some money for it either through ransom or "claiming the reward" for "saving" them.
Keep farming those diplomacy rolls all around taverns to get whatever news you can.
Meta-Analysis of the situation
This isn't meta-gaming, this is your characters thinking "what do I know and what do my enemies know and what do my enemies know that I know?" particularly, your enemies may know a doppelgänger of your friend is being used, do they know the doppelgänger is dead? They know what the doppelgänger should look like... your friend. Do they KNOW you know that?
Is it within the scope of your abilities to then use Disguise spells of some sort to appear as your old friend but then still pretend to be the Doppelgänger. It's the perfect disguise, any spell to detect magical disguise will of course be detected as if should... you are supposed to be a shape-shifter.
You can even walk in acting as if you have tricked your party and are "luring them into a trap".
Would it be possible to go where you'd expect your Doppelgänger to go and wait to be picked up by their contact and work your way back from there. It would be a great counter-play, you'd at least learn a thing or two. Bring up that you need a vital piece of information from the person you (posing as doppelganger) need from the person your are impersonating.