"She was happy only with her."
Translation:Elle n'était heureuse qu'avec elle.
The position of qu' was slightly different: https://forum.duolingo.com/comment/40115224/She-was-only-happy-with-her
"Elle n'était qu'heureuse avec elle. → She was only happy with her.
Elle n'était heureuse qu'avec elle. → She was happy only with her."
Except the two English sentences have identical meanings: she was happy with nobody else.
It's scummy enough to give us pairs of sentences with only nuanced differences. It's absolutely despicable when the two sentences have the same meaning and are yet marked differently.
It's the difference between "she was only happy (when she was) with her", implying unhappy whenever the 2nd woman wasn't around and "she was happy only with her" (and with no one else). In french the position of the que clarifies the difference between these two meanings, similar to where the "only" is in english