Actually, I have dealt with this situation before in two different scenarios.
If she was a kid wondering into the yard for a ball and the dogs bit her, she would be at fault and considered trespassing.
If she was not an employee and went into the yard and the dogs bit her, she would be at fault for trespassing.
However, she was an employee who had access and unless she was there to commit a crime or broke in, it will be nearly impossible to prove trespassing.
The reason I say "unless she was there to commit a crime or broke in" is not a blanket statement for ALL people, but for her, an employee, specifically because that would negate her authorization as an employee.
If an employee goes to their office at midnight and has a key, that will not be considered trespassing unless they committed a crime while doing so, though the employer could fire them for doing so if it violated procedures. However, if they went to the office, broke in and then caught up on some work, that would be trespassing because they committed a crime in gaining entry even though they were there to actually work.
Not following procedures is grounds for termination, but not grounds for relief of liability.
Now, if she had been told not to return to the property and did so, then yes, that would be trespassing. However, simply saying, "Yes, she was allowed but only when the moon is full on a Tuesday" is not enough to remove liability from an employer. If that were the case, then every employer could simply say, "I told them not to come in the day they were hurt so they were trespassing" on every work-related injury.