I believe that the decision to give Alex an allowance was influenced heavily by the amount of public appearances she did in the years leading up to the divorce. If the circumstances have changed, then yes, I think the allowance should be reviewed.
Why do I think the circumstances have changed? Well without further information (e.g. patronages saying "yes, she does a lot of work with us behind the scenes), the only thing we have upon which we can base our decisions on is what we have available, including (i) the number of patronages (the assumption being the more patronages, the more work there is), and (ii) the number of public appearances. Reduction of public appearances and reduction of the number of patronages is likely to make me think "yep, allowance should be reduced". Its only reasonable.
Giving such a large allowance to a soon to be private citizen is not an absolute right - therefore the onus of proving that the allowance should continue to be given is on that person, IMO. It should not be up to me to prove that she has not reduced her work.
But then again the Danish government gave her the allowance without reference to what it is exactly she has to do to continue receiving that allowance. So perhaps its possible to do whatever it is she wants to do and still continue receiving that allowance. The Danes are very generous people it seems.