Doesn't that sentence imply that she was thinking about someone else's mother? As opposed to «Девушка думала о своей матери»?
It depends on the context. Yes in some cases this sentence may mean that she was thinking about someone else's mother.
Why is "the girl thought OF her mother" not accepted?
In Russia woman can be young or old, doesn't matter. Without young is correct too
Why does this not accept girl as an answer? Young woman isn't what i think if when i think of девушка
Why devushka HAS to be a girl/young woman? I know it differs from zhenshina, but is it so distinctly different that it's marked as a mistake?
Shouldn't it be "об её матери"?
Some Russian vowels have a bit of a y sound at the beginning, depending where they appear -- е "yay", я "yah" and ё "yo". Those are "consonant-like" enough to not need the б on об.
No, it's correct here. But if she was thinking about her own mother I would say "о своей матери"
Is матери here in prepositional case?
'The girlfriend was thinking about her mother' was not accepted. Why not?
Whose girlfriend? Without specifying whose girlfriend it can be (like, его девушка) it can be only "young woman" or "girl".
Думала о СВОЕЙ матери.