Apparently he copy pasted out of the wrong text file.
Thats likely, but why did he have that textfile with addresses? Usually because you know someone with that address.
Well you can store you BTC at your CEX.io wallet at the address they provide you.
You can, but you should not.
Im not sure how you even got to the part about owning all of CEX.io. Its one wallet receive address.
A wallet -at least for me- is something that gives you control over at least one private key. So if anything happens to the service you still have the private key to spend the funds useing another wallet.
Cex.io is a service that has a balance. If they go away tomorrow your friend (and everyone else that though storring coins there was a good idea) has nothing but usless login data. The address your friend sends money to in order to increase his balance on cex.io is not under his control. Thus following the balance of that address is irrelevant. The only thing that is relevant is the balance on cex.io. Big companies will not store the money you send them on the addresses they give you to pay.
Yes that is true. Its not an actual wallet like Armory or a generated cold storage wallet.
I wouldnt leave my balance on there but he chooses to use it. Not what I would do but that is on him.
Seriously, I dont know him well enough to know where the address / file came from but I think he was
not in the correct state of mind to be sending funds of that amount in that state.