There's really nothing for YOU to really do. The current owner needs to take responsibly in getting a trainer or being the pack leader.
However, you're looking for suggestions
What if you offered to walk the dog occasionally? Not only would the dog benefit from the increased exercise, but then it would begin to recognize and feel more comfortable around you. Once she feels she can trust you and possibly look up to you, you can then move on to meeting your dog. Maybe she just needs an authoritative person in her life. I mean after all, GSD's really look to their masters, and she's obviously lacking one currently.
Not really an expert though. There are some awesome trainers whom are on this site. Hopefully they'll chime in!
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