How awful
Give my regards to her and her family.
As for keeping folks safe - the buddy system is definitely the way to go, but I understand that it's not always going to be possible due to budget constraints. I'd second Jonathan and make sure that it's part of the training program, insofar as knowing how to be aware of potentially dangerous situations and having extra precautions in place for lone employees. Heather's also got some great ideas, particularly about going with your gut. Listen to it!
The thing is, though, to be careful about how you create your precautions, because you absolutely never want to include language that may make the survivor feel at fault, or like s/he didn't "do enough" to prevent the assault from occurring. They'll get enough of that from everywhere else. Make sure that you know your local rape crisis centers and have their telephone numbers available. Have a support system in place - external will probably be necessary, but make sure you have access to it and that everyone knows how to make the best use of it.
And just my two cents: you may want to consider keeping the morning-after pill in your office medicine cabinet. We all hope that rape isn't going to be common enough to warrant it, but there's nothing like the terror of having been raped with the nightmare of possibly being impregnated by your rapist, and not every hospital/nurse/local pharmacy will provide it.