Sorry, but it's wrong to assume that a probationary period can't be extended. It depends upon the terms of the contract.
Extending a probationary period is often in an employee's favour. If it wasn't possible, an employer could be forced to decide between keeping an employee who hadn't yet met the required standard or sacking them (despite the possibility that they might soon 'come up to standard'). Faced with just those two options, any sensible employer would probably decide to terminate the employee's contract.
Extending a probationary period allows an employer to 'take the middle ground' by giving the employee a second chance. Within certain professions (e.g. teaching) it's quite common to extend a probationary period before full professional status can be achieved.
If your daughter's contract states that her probationary period is assessed purely on a 'pass or fail' basis, and she's not actually been failed, then the probationary period can't be extended. (That would seem to be the type of contract that Meglet's colleague had). However, if her contract states that she'll only pass her probationary period after a satisfactory 3 month appraisal, and offers the possibility of an extended period of probation (rather than dismissal) if she fails to meet the required standard, then everything is in order.
Chris