I've had exactly this issue in the past - worse in that the client was also arranging for third-party design and UX, and we were just to put together the techni

I've had exactly this issue in the past - worse in that the client was also arranging for third-party design and UX, and we were just to put together the technical solution. Honestly, the best you can offer is advice - it's worth highlighting any particular problem areas that you may be able to help solve as part of the technical process. However, at the end of the day, this guy's probably going to go ahead and build this app regardless of whether or not you agree to take it on. So the choice is a) it doesn't work, and you get the money or b) it doesn't work, and someone else gets the money. Sure, if there are going to be long-term ramification for you, of the "Hey, aren't you that guy who developed that awful website" kind, then maybe think twice. But it's realistically not your job to validate business ideas, and it's better not to even start down that road - because you'll end up sharing responsibility. Just make sure that you're paid before launch!