Hm it's tricky, it depends on what kind of person she is...
Could you make her a lovely album? Like with pictures of her when she was young and growing up (if you've been friends for ages!) and in her adult life to 40 like getting married/having children (if she does!) or whatever means most to her. If she's having a party you could always get everyone to secretly write a favourite memory or something lovely about her and put that in it, or in a book - someone did that for my mum's friend and she was delighted.
Or take her on a special day out or a trip together? It's my mum's 50th this year so I'm taking her to london in a couple of weeks for afternoon tea, harrods etc...
It's a tricky one, good luck!