If you activate a new phone on your account you can get the insurance on it. It even works of you remove the phone, and activate a different phone on that same line, then move the original back on it. I know as every time I swap out phones, I am asked if I want the insurance through Sprint. Even when I add a old phone I keep as a backup. I use best buy for insurance coverage so I don't take Sprint up on it.
Lets hope nothing happens to your phone anytime soon. Insurance company's have policies in place to be able to reject a persons insurance claim if they have to many in a certain time frame.
So you file a claim on your buddys phone, a month later your phone crapps our or you break it, file another claim, its that 3rd claim in a year that they start to say, nope sorry you have filed to many claims and we are no longer carrying you.