Its probably something that varies from council to council.
I owned a flat and was living (actually moved in) with my partner 60 miles away for 50% of the week. Hecontacted the council and we were told it was OK because i was also paying council tax on other property, but they did say that it has to be declared to them
It doesn't matter if you are paying on another property or registered elsewhere on the electoral roll - if you can be determined as residing there she will lose the single person benefit. A couple of times a week max is a rule of thumb. If its the same days every week that will look that a residential arrangement though, so be careful.