Although, I haven't repeated a subject, I would think it would get quite boring and the student may not work as much as they should because they would mostly know the content of the course. (May not be true)
Is she particularly good at any other subjects?
For the repeat to work well, she'll need to beat what she could've achieved in a 6th subject by ~2.6 as to receive the same aggregate.
So, if she thinks she can maintain motivation and be able to score well, then by all means repeat the subject.
I've seen success stories from repeating subjects - werdna boosted his English study score up from a 34 - 46 (or similar) - so something similar may happen for your friend.
Personally, I wouldn't, I'd have a go at a different subject and aim to score highly in that (but she may not have extra subjects in hand that she could score highly in)