Tell him that SQL is designed to use all the memory it is allocated and that using 90% of the memory is a good thing.

You don't have a problem (except for a client that appears to refuse to believe you). you are seeing normal, expected, intended behaviour.

It's like saying, "My car goes fast when I press the accelerator, what's wrong?"

Now, we've repeatedly mentioned max server memory, and a chapter in a book all about memory, so by now you should be in a good position to know exactly what (not) to do in this situation.