The problem is that instructions are subjective unless they were written down in a signed contract detailing those instructions.
However, even a signed contract would likely not be enough to justify trespassing unless she was there to commit a crime. Now, a signed contract with detailed instructions could possibly be used as an argument against liability, but even then if it says something like, "let employer know" that falls more under a reason for termination, not a defense against liability.
For example, if your employer tells you to take a day off and you show up anyway and break your leg, your employer is still liable for it because you are still employed by them.