The employee has not objected to the employer’s action, so has (by not objecting) accepted the action
That’s where the employer is in breach
But applies equally well if I change the wording round
The employer has not objected to the employee’s action, so has (by not objecting) accepted the action
The sentence that you have posted is awkward in that it means that, if the injured party waives their right of action that arises as a result of the breach, the injured party has (by failing to take action) accepted the breach and, by that acceptance, has affirmed the contract