Student or Learner
"When they moved to France, however, she became/has become friends with a Swedish neighbour."
Is the present perfect also possible in this sentence, because the action began in the past and is still continuing in the present ?
No, because the 'becoming friends' is located in the past time - when they moved to France. If they are still friends, and the sentence doesn't tell us that, it's 'being friends' that has continued to the present.