Get paid or Open Source the code
Any time a 'business interest' decides to establish legal restrictions I would absolutely make them pay for that privilege. A NDA is basically a gag order that prevents you from sharing internal information pertaining to their company/project. Basically, if you talk about information private to their company they have the right to sue.
It's insurance for them but if they decide to lock the code you wrote into a proprietary licensing scheme there's nothing you can do about it as talking about the internal details would be violating the NDA.
If you don't already have the code covered under a license, you could try to release it under a permissive license (ex MIT) that will guarantee your right to continue using the code you wrote in the future.
The only problem with retroactively applying a license to existing code is, every contributor needs to agree to the license terms as their individual contributions have an implicit copyright to the developer who wrote them.
Worst Case (they win, you lose):
Your friend jumps onboard, they use your code, and lock it down under a proprietary licensing scheme.
They can legally prevent you from using the code you wrote on future projects, unless you lawyer up and come up with sufficient proof that they are using your code without permission (ie violating your personal copyright).
Either way, if they go with a proprietary licensing scheme you should be compensated for your efforts.
Good Case (they lose, you win):
You setup terms for compensation for the code. Basically, they purchase rights to use the code you wrote without restrictions. At that point a NDA doesn't matter because you lose rights to build on the code anyway. Just don't share any of their internal information publicly or you put yourself at risk of being sued.
Best Case (win, win):
You release the project under a MIT license so anything written up until now is free for everybody to use and build on (including for commercial purposes). It guarantees that you can still build onto the code in the future regardless of what they use it for. A NDA still doesn't matter as long as you respect the terms. If they want to go in a proprietary direction, they're welcome to fork the main codebase.
There's a good chance that they're just trying to protect their interests. There's nothing wrong with that but be careful not to give them the legal right to screw you in the future if they choose to go down a path you don't agree with.
Even software that you write pro bono still isn't free until you setup a license that guarantees future usage. Sweat equity (ie effort) is always holds value and as long as it can be used to generate profits, there will always be somebody looking to gain ownership over the rights to use.
Some terms may depend on where your live too. Intellectual property rights are a messy business.
Disclaimer: IANAL (I Am Not A Lawyer).