@bugsonmydesk she totally deserves it! I was going to suggest something else entirely but wasn't sure of your budget.
The thing with Stella McCartney bags is they're expensive but not particularly sought after
. Same with Jimmy Choo bags, Anya Hindmarch, Tory Burch, Victoria Beckham - their resell value is depressing (have a look on eBay). If it's a bag she's going to make use of everyday and run it in to the ground then fine as she'll get her moneys worth. If she ends up storing it and saving it for occasions she will find the style has dated and then it will just sit at the back of the wardrobe forever (the Falabella was launched in 2010).
Would she be open to a more premium brand classic bag from the pre loved market? (As in almost new condition - Vestiaire is a good place to start ). It would serve her well in the long run. You might even find the Dior bag she wants or the much sought after Loewe Puzzle bag 😉.