He's going to have to set up a new ID. That old iD is registered on the French store and can't be changed. Likewise he wont be able to use that email address on the UK store. I've had these frustrations myself between a couple of IDs but there it is. The good news is that if he wants anything from the French store, he can have it (as long as its free, or he can supply a French CC).
But... a new AppleID is going to be needed for the UK store.