I would avoid saving it as a different file type etc - you will run into all sorts of problems.
You can simply drag the photoshop layer from one image into another. If you need to move all layers then I think you can select them all using shift or ctrl and drag them all across.
Yea or that, I was suggesting my way to him incase he would encounter unwanted layer lighting, colours, effects etc on things he didn't want them on.
So thats why I said maybe something like a .png so its just one layer