Is there a genuine reason why it's late? Wages /housing benefit payment? I know it should be paid on time but I bet if you acknowledge you are accepting of the new date the tenant will be very grateful (and you will feel all warm inside for making someone's life a little less stressful!) As a tenant myself it brings a sick feeling to my stomach if I know my rent was going to be late. It isn't these days but in days gone by it happened. .
It sounds as if she hadn't understood how a standing order works actually. I used to work in letting sand initially new tenants were always surprised that the standing orders we set up were a few days before the rent was due. God knows how in this day and age, but standing orders still seem to take a few days to get from one place to another. I thought you meant weeks late, a few days late would seem to indicate that it's a banking misunderstanding on the tenants part.