The router will have a different IP address that way you're hiding yours. If a person hacks you, all they're going to get is the router and not your personal files.
How exactly is this going to happen? The router should have the same ip address that you normally have, it just tranlates it to a different one (usually non-routable) and forwards the traffic to the correct box behind it. Not to mention if someone is good enough to get into your router I doubt they are going to have much of a problem looking at the router's logs and seeing what machines have been assigned what ips, and wham, they are right where you think they couldn't go.
Listen, asking others to hack something for you (even if you don't want any damage done) is often considered bad form. If you already know that the security is poor, and you bring this to their attention, and they don't do anything about it, then so be it. Learn how to and then protect yourself. Don't rely on them, even if they were good at it. It really is your responsibility anyway.
Good luck