Bad app ideas

My friend is trying to convince me to work together with him (I'm a programmer and he struggles with basic HTML or even English grammar) for an app. He has a terrible app idea that is too complicated and won't be profitable either. He has no idea what he's talking about but he is being very pushy to try and get me to commit to working on it with him, and he basically would want me to do all of it and then we'd split it. What a fucking stupid idea.

How can I tell him off without being rude?

Lure your friend to the Saudi Consulate under some pretext. When he arrives, overpower him and cut his fingers off, one by one. Then decapitate him in the name of Trump and Jared Kushner. Put your earphones on for some peace and calm while you use the tool of Mister Bone Saw to dismember his body into disposable pieces. MAGA (Make America Great Again)