The key issue is: do you have ethical and professional obligations to a potential client. Or only to an actual, contracted client. Or not even then. Since you'v

The key issue is: do you have ethical and professional obligations to a potential client. Or only to an actual, contracted client. Or not even then. Since you've posted this question, you presumably agree with the last two propositions. So, you believe that you have knowledge that the client does not. And the lack of that knowledge will cause him to make a mistake. I would say that you have a professional obligation to advise him that the webapp is unlikely to succeed, but that this should not be used to override his freedom of action. If you explain your concerns and he still wants to proceed, then take the work. The degree of any ethical obligation you might have depends on the harm that might be done to him by his failure. If he is wealthy and can absorb the cost of failure then, in warning him, you've probably discharged your ethical obligation too. On the other hand, if the client has re-mortgaged his house to pay for the app and he and his family would end-up homeless when it fails then I'd say you have a duty not to involve yourself. Obviously, the grey-area in-between is tricky. If you don't think there is any ethical component to this then your decision will presumably be easier. I hope that is not the case.