The Project Managers know there are other options that will work better.
The implication behind this sentence is that the PMs know with absolute certainty the other options will work better. If my inference is right, I find that implication odd as I am not sure I have ever experienced a time when a complex problem had a solution that was guaranteed to work better than another solution. Guarantees are best left with the sales and marketing people.
Every solution to a complex problem has its fair share of benefits, costs, risks, and penalties tied to it. If you think there is a better solution than your client's, build and present the case and an non-emotional, objective way. Present your findings for all potential alternatives on all four of those things. Give you recommendation and let your client make a choice. Then implement his choice with the professionalism he needs.