if he was still running the stock pre-cats it is NOT recommended to place them after the pre-cats. You want an unmodified/unobstructed reading. If your running down pipes/test pipes yeah you can put them farther downstream because its nothing but pipe between the sensor and turbo. But if you are using stock exhaust or pre-cats it needs to be before any catalytic material.
So since he decided to upgrade the pipes its really up to him. He can put it in either place just as easy during the upgrade.