> 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.