Hi J.,
We are dealing with a little of this here too. I do like Cheryl's idea of the gate at her door. There are alarms that you can put on the doorknob or you can just put bells or something like that as a way to wake you up.
I believe it has to do with sleep cycles, so you may want to track it to see if it happens around the same time each night. If so, you may want to wake her up before it happens to break the sleep cycle and sort of "restart" the process.
With my son, his sleep walking is combined with night terrors and it happens between 12 & 1. We wake him up around 11 before we go to bed and that has seemed to help.
Good Luck!
B.