In my experience CSA try and get you to sort it between yourselfs, when contacted by the CSA your friend needs to tell them that she already as an agreement with the childs Aunt and is willing to stick to the agreement, they are then likely to ask your friend to put this into writting and will contact the Aunt to make sure she is happy with the situation. Any nogotiations can be taken from here. If the aunt still wants it to go through CSA then they are likely to take upto 15% of her wage (not all of which will go to the child, and the Aunt needs to be aware of this}. It would be best if the 2 adults could talk and come up with a solution between them selfs the CSA do not like to get too involved but when they do both parties can end up worse off. If I was your friend I would point out that if she was to go through CSA then she could end up getting other benifits reduced as they do take everything into account, They knid of give in one hand and take with the other.
In the mean time all your friend can do is keep a record of any money she gives to the Aunt, the best way of doing this is to put it into a bank account which is in the Aunt name and keep bank receipts etc, that way it can never be said that she as never give any money
Good luck to your friend