Am I allowed to order a gig from a friend and leave them a review? They did work for me and it could be added to their portfolio.
No problem until you sell the same service/file which you got from your friend.
If you order, they complete the order successfully, and you leave a review, there’s nothing wrong with that. How would Fiverr know that you two are friends?
If you do it too much, it might raise a red flag, though.