Can an employer hire someone else while the employee is on leave?

Yes. Just because a role is job protected, that does not mean that an employer needs to leave that seat vacant, especially if the employee is gone for longer than 12 months. It simply means that the person who is off needs to be able to return to the same (or an equivalent) seat upon their return. 

Industries such as teaching are largely built around contracts for employees filling in for absences. It is, of course, more difficult for many private companies to find a suitable employee who is willing to accept a job for a finite amount of time. This can be even more challenging in specialized roles, where the pool of available talent may be further limited.