This could definitely stand to be expanded on (we're planning on it in SUPPLY DROP), but how we've run it without specific rules is pretty much treating the allied NPC as an extension of the PCs, letting them land a shot when someone rolls a success, letting them add a +1 to player rolls using the optional rule for advantage and assistance, etc.
Oh! Okay, Thats interesting, So kinda like how enemies get a surge, Like tossing a grenade when Players Complicate, When Players Succeed their Allies should get a surge against the enemies?
Thats... A nice way to run it, Keeps the spotlight on the players, but keeps the allies useful, Pair that with letting people give them orders as the "theme" of an attack action where they use that NPCs weapon instead of their own, And i can see how that'd work out to be quite fluid!
Besides the NPC Allies also being an extra pool of HP to drain from.