The Project Managers know there are other options that will work
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.
EDIT: How to convince? I think it becomes a sales call and, as with any product you are trying to sell, it becomes an argument that shows how the benefits of one product over another fulfills a client want or need. You need to be able to substantiate that the features, advantages, and benefits of your recommendation meet your client's needs and wants better than the other alternatives and that the costs, risks, and penalties of that recommendation are appropriate and less than the material benefits. I can't break this down into something more tactical without knowing more of the specifics.
It is less about you convincing the client and more about the client realizing a better choice. So the truly best alternative should sort of pop out in your business case.