Hey,
You need to continue playing the "Guardian Rises" campaign, the XP system unlocks toward the end, during the "Advent" story mission. Once it's unlocked, you should be awarded all the XP you've earned so far.
It's probable your girlfriend had previously completed the campaign with a different character, as the system unlock works per account, not per character.
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Thank you for your answer! :) It works now!
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Excellent, glad it's working now. Have a good one.