" Hun er veldig flink til å kaste ballen med den høyre armen sin, men den venstre armen hennes er ikke så god."
English Translation:
“She is very skillful at throwing the ball with her right arm, but her left arm is not so good.”
“Hers” in English usually translates as “hennes” in Norwegian, so why, therefore, is “her right arm” translating as “den hoyre armen sin,” but “her left arm” translates as “den venstre armen hennes?”
In English, you usually say “Her left arm”, not “Hers left arm”. Both “Her” and “Hers” will translate into “Hennes” because you are referring to the “Left arm”.
Both “Den høyre armen sin” and “Den høyre armen hennes” are accepted.
I’d say it’s quite common in Norwegian to use “Den høyre armen sin” as said in the example you gave, because the sentence continues with “men den venstre armen hennes” ~ "But her right arm.
It’s complicated, I know, but for the sake of keeping things easy - both terms are accepted.