Like Nancy Reagan Used To Spout; "Just Say No". (Score 4, Insightful)

How about just a simple "No." The database, while containing data pertitnent to your customer, is still your\your companies property. Simply tell them that access to that level, or in fact any level beyond what is alreayd granted to them as a customer, is for you and\or your employees only. Just because he's a customer, doesn't grant him unfettered access to your company or it's property, whether that property is physical or electronic.