I still think you're best off creating a second listing!
My suggestion would book all 6 months *on the long-term listing*, which would only be booked by her long-term guest. @Sara71 would keep her original listing, just manually blocking off the mid-week stays & allowing other guests to book the weekends.
The problem with having the long-term guest book a number of short stays is that those will fall under your flexible cancellation policy, and could be cancelled at any point with little ramification to the guest. That's a significant risk when you could be turning away months of longer (non-weekend only) stays.
Going the single, long-term reserservation route, you're much more protected by the strict long-term cancellation policy, the guest only needs to make a single reservation, and you're still able to book weekends for other guests.