In my view this is typical of a poorly managed organisation. Many years ago motivational periods had some relevance but pro-active line management should be helping to integrate a new employee into post anyway. To say, you've got to prove you are worth keeping is akin to admitting they don't trust their recruitment processes.
But as F2F points out, every employee can be dismissed without reason, save for the discriminatory ones, within the first two years, so I reckon it is better to keep one's head down and get first two years done.
Disappointing her shift has been changed. Is she in a line of business where no premium is paid for night working? Also employees have to observe rules for employing young people on permanent nights, as well as need for health checks. Can't remember all details right now but can be checked.