I wouldn't buy moisturiser because by 40, she's probably got her day to day routine pretty sorted.
How about -
A Lancome mascara; Clarins Beauty Flash Balm; A Chanel nail varnish in a neutral colour and just one really lovely pair of silk knickers; A big bottle of 'posh' shampoo and matching conditioner - eg Aveda.
These are all things I'd be v happy to get, anyway!
An expensive bottle of champagne, choclates and strawberries. She can share them with you and enjoy the milestone, talk about the good times and celebrate and be grateful for what you have accomplished.