How would you convince them to abandon their plan to dive into project management and use an existing solution?
List all the reasons you think it's a bad idea, present it to them, let your opinion and advice be heard. Then tell them you will go with whatever decision they go with, it sounds like they have already made up their mind, if you are getting paid, just do it. (If you're not getting paid or just want to help then mock up an currently existing solution and show it to them as proof of concept.
Hey, people/companies/government's make bad decisions everyday and there is usually someone in the corner (waving their arms or not) who actually has a much better solution, who was unheeded.