i'm with mike, it seems under to me I just used a (paye) tax calculator, and someone earning 46 000 PA should pay about 9000 Tax in that year, around 9.5k for 47 000 and around 6 k for the 36000 year which to me adds up to about 24.5 k she should have paid just for the three years you've given us, let alone the other year you haven't given us.
Of course, a self employed person probably gets tax breaks too, so it's simply impossible to tell Perhaps if she is worried, she should employ an accountant?