First, the 6% deferral is part of their salary and a piece that they have control over on a payroll period basis. So if the total negotiated compensation is $100,000 (forgetting about the match for now), the $6,000 deferral does not reduce compensation for other contributions and benefits, it only reduces taxable compensation.
Employer contributions are not considered compensation so there can be no employee election, otherwise you have a cash or deferred arrangement (i.e., more 401(k) deferrals). However, as part of initial negotiation and setting starting compensation, I see no reason an employer could not assume the employee will defer 6% and get a 6% match. Say starting pay would be $106,000, and match of $6,000 is assumed in the total rewards package such that compensation is started at $100,000 - and that should be locked in, if they decide to not defer (or to stop) they don't get that match in their pay.
I think a dig into the 401(k) regulations and definitions of what is a salary deferral and what is a matching contribution will clear that up. A match being an employer contribution contingent upon the employee making a salary deferral. Your client wants to also make it contingent upon/resulting in a reduction in pay.