I am a bit confused here as to what the levels are we are talking about.
Let's assume there are three levels: Level 1 is a master standard, Level 2 is a standard calibrated by a level 1, and level 3 are the instruments being calibrated by the 2. Level 1 is approximately 4 times more accurate than level 2, and level 2 four times more accurate than level 3.
The same level should not be used on another instrument of the same level. So whichever one you are shipping off (your equipment used for calibration), you need to "borrow" equipment at that same level, or higher level.
So I would recommend against using a level 2 to calibrate a level 2, and so forth.
Now... let's assume you are using a level 1 or 2 to calibrate a level 3; and the level 1 or 2 is your customer's unit. I would say it is OK, as long as you can provide evidence that the customer's unit received some kind of rudimentary check prior to use, that it was OK to use it instead of your regular standard (procedural), and that you have the ability to assure the unit was fit for use while you used it (as found and as left data). So, if the unit was calibrated in June, and you used it for your use until November. Prior to returning it to the customer, you would need to have a calibration performed to assure that it was not out of tolerance while you were using it. You will want to maintain records of the calibration of the unit that you used for calibration.
Assume you have the proper accuracy ratios and that you have the other stuff covered, then the question (possible ethical question) is whether it is appropriate to borrow the customer's equipment. I would ask yourself "how would you feel if someone was using your standards in their shop?" Not saying it is right or wrong, but I would consider that, and let a good, ethical decision guide your actions.
