If the owner is standing there with you for 100% of the duration that you work on his stuff, there's no issue. In that particular instance you do not have to be in possession of an FFL to service guns. As mentioned, it becomes an issue should the firearm be left in your possession overnight. If he leaves, the firearm must be in his custody. You can stab as many barrels onto receivers as you want so long as you adhere to that rule. The BATF won't give two chits about it. That is not manufacturing. It's customizing, altering, tailoring, whatever you want to call it.
Manufacturing is you bring in X product, work improvements to it, then turn around and sell it for profit. Example: You work over a dozen M700's, stab barrels onto the action, stuff em in a stock, and flog em out the door as "acme whatever killin machines". Now you just stepped into manufacturing and all the rules/requirements become relevant.
Stabbing a stick on neighbor Bob's 308 doesn't qualify as that. You customized his existing product. You provided a service, not a product.
From Uncle Sugar's viewfinder its about like sticking a holley 4 barrel on a car.