You'll be grateful of your mate's input in doing the work together if he's an experienced spark, particularly as your OP suggests this will be your first CU replacement. It's pretty much guaranteed you'll run into issues and have to make decisions along the way, which is where his experience will be invaluable and you'll learn a lot from him. A replacement of something old with short tails, in a less than ideal working position, when you're up against time getting power back on, is very different to doing it in the controlled environment of a spacious, well lit practice booth with new cable.