I Cannot see why it won't work, as all you do when fitting a new ecu is program the keys into the ecu,
so, fit the new ecu, swap the transponder chip from the new key to the original key and it should run.
otherwise if you have a half decent diagnostic tool, and he has more than 1 key, have a go at programming his original keys back in. ( assuming that it has the option of Timed Access )