Should a custodial parent expect a non-custodial parent to enforce their punishment?

Earlier in the week we caught our youngest son in a whopper of a lie. We had caught him in some smaller lies previously and this one was not only a real dozy, but he told a second lie to cover his tracks. He was of course disciplined every time it happened, and because his habit seemed to be escalating we brought the hammer down pretty hard. For this entire week he is grounded from playing with friends and from his ipod, he is also not allowed to play outside or watch television. This is the evening he spends with his dad. The most we could do was make him leave is ipod at home and explain that since we have no authority at his father’s house we can’t enforce our punishment there and it is up to him to do the right thing.

So here’s the question, is it unreasonable of us to expect my ex-husband to enforce our punishment? Should he back us up for the good of teaching the boy a very important lesson or should we leave our punishments at the door when he heads to his dad’s? What is your opinion?