Although the WTD is not that enforced it would flag up on a check by who ever and as conforming to the legislation is all part of the o licence agreement it could well cause problems for the company if not addressed.
If the driver is simply not worried because he’s an agency driver I personally would let him go. I’ve had 2 infringements in 6 years both to incorrect tacho mode where I’ve turned the ignition on and not realised the mode had changed.