>>>  your customer doesn't have a clue (I know that you can't tell them that) but it's true.


Not necessarily true.  In a former life I dealt a lot with high frequency RF assemblies.  In this world of black magic and chicken blood, we all know that the PCB is no longer merely an interconnect, but rather it actually becomes a component of the circuit.  In fact, many times components are created solely in copper (capacitance, inductive couplers, etc).

What's more, in RF (maybe AC/DC too?) the signal propagates primarily on the outer skin on the conductor. 

And, SnPb is more lossy than Ag.

This is not proof that the supplier knows what he or she is doing, but they may have a clue.

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