In the FIDIC Silver Book (edition 1999), if the Employer has instructed the Contractor as...

This statement is true. Under FIDIC Silver Book 1999, Sub-Clause 8.6, the Employer can instruct the Contractor to submit a revised programme to meet the contractual Time for Completion. If, after such instruction, the revised programme still shows the Works will not complete on time, the Employer may claim additional costs (such as delay damages or compensation) due to continued delay.

Thus, the Employer’s right to claim additional costs is contingent on the revised programme not enabling timely completion.