Kris suggested leaving out you, me, I terms and discussing facts rather than personal opinions, and also suggested that you present your alternatives starting with Consider.... Those are good suggestions, but it might be better to begin with a statement of objectives (if you know them) or a question about objectives.
For example, any of the following might be the most important concern: efficient operation; timely implementation; long-term flexibility and adaptability; immediate or lifetime cost; reuse of existing resources; or correctness and completeness.
After objectives are clear, then one might point out where the client's suggestion falls down on one or more primary objectives, and briefly mention that methods exist that meet the objectives. (If the client's suggestion does not fall down, there might be no compelling reason to suggest an alternative.)