In principle 'Yes' I would provided the Contractor is acting in accordance with clause 63.7 of the NEC3 Engineering & Construction Contract - I assume it is NEC3 and not 4 by your use of the word 'Employer', but the equivalent clause is in NEC4 ECCis 63.9.
NEC3 clause 63.7 says:
"Assessments are based upon the assumptions that the Contractor reacts competently and promptly to the compensation event, that any Defined Cost and time due to the event are reasonably incurred and that the Accepted Programme can be changed."
I.e. they are actively trying to source an alternative subbie.
In addition, they have a duty to mitigate in law.
The one proviso to this is that if the Subcontractor has already started work on that bit of work then, even though they have not signed the contract, they have effectively accepted its terms by conduct. But if the Contractor is doing its best, I would not punish the Contractor for that.
NEC3 clause 63.7 says:
"Assessments are based upon the assumptions that the Contractor reacts competently and promptly to the compensation event, that any Defined Cost and time due to the event are reasonably incurred and that the Accepted Programme can be changed."
I.e. they are actively trying to source an alternative subbie.
In addition, they have a duty to mitigate in law.
The one proviso to this is that if the Subcontractor has already started work on that bit of work then, even though they have not signed the contract, they have effectively accepted its terms by conduct. But if the Contractor is doing its best, I would not punish the Contractor for that.