NDA keeping me from helping an existing startup?

A friend recently asked me for help on an existing startup. It's basically an indie startup---no VCs. So I would continue at my current job. But since I'm not a lawyer, I'm not sure if the NDA is keeping me from helping out. Can someone point me towards some legal resources?

So to be clear, someone else invented the app (and it already exists). I would just help code new features. My take is that because the app was invented by someone else that the NDA doesn't apply---it specifically says I have to give up "copyrights, patents, etc" of any "inventions"--- but I can't give up something I don't own, right?

In a related vein, exactly what is an "invention" in the eyes of the law? If I create, say a TODO app, does that count as an invention? Does a book of fiction count as an "invention"? Where's the line?

Here's the NDA boilerplate.

I hereby assign, and, to the extent any such assignment cannot be made at the
present time, agree to assign, to the Company all my right, title and interest throughout the world in and to all Inventions, and to anything tangible which evidences, incorporates, constitutes, represents or records any such Inventions. I agree that all such Inventions shall constitute works made for hire under the copyright laws of the United States and hereby assign and, to the extent any such assignment cannot be made at present, I hereby agree to assign to the Company all copyrights, patents and other proprietary rights I may have in any of such Inventions, together with the right to file for and/or own wholly without restriction United States and foreign patents, trademark registration and copyright registration and any patent, or trademark or copyright registration issuing thereon.