Sleep walking is sometimes genetic...I wonder if the other dogs in her family displayed this behaviour too. Unfortunately, there's not much that can be done for sleepwalkers (human and canine); some people do try medications such as anti-depressants with some success, but a lot of people find the side effects of anti depressants unbearable too. Others have tried various sleep inducing drugs, or setting their alarms to wake them up just before their usual sleep walking time, in order to disrupt their sleep cycle.
It seems as though both Bizkit and her owner have learned to live with it though, so long as there's no underlying cause for the sleepwalking (e.g. tumor, epilepsy). I'm personally not a fan of medicating, I think drugging the dog to stop the sleepwalking would be riskier than the sleepwalking itself (so long as stairs were blocked etc)