If your friend cannot give you a fair equity stake, its not worth your time and effort to make it work. Before signing the NDA (which is perhaps just legal due diligence), you should have been equally formal to make equity a pre-condition of signing, that would have been a good lead into negotiations. Or, maybe walked away. It sounds like you surrendered that opportunity because you're a nice guy and this is your friend, but..

This is business. I think you need to be honest, you feel like a "regular employee". Friends or not, you're going to be a sharecropper on your friend's land, and that's going to ruin your friendship if you keep quiet. What makes it better than any other gig? -- you said it yourself, you might as well freelance on honest terms up front as a contractor or as a regular employee at a regular company with money to pay you. Why should you risk investing your precious free time?