are you sure that that is the figure that your aunt is being asked for and not the total works?
Does she pay a monthly fee already? The management company for my flat costs me �50 a month (very very expensive for what is is) but we are all directors and get to see the financial accounts and decide where the money is spent.
Check her lease and what it says about maintenance work
You need to find out exactly what shes being asked to contribute and what the accounts already hold. Ask to see copies of the quotes for work if necessary as this does seem to be an extortionate amount of money to ask per flat just for new windows and boiler.
Just to add, if she does not pay to the maintenance company, depending on who runs it, then the lease can be taken away from her. This, in theory, means that she will have no right to live there and will forfeit her flat.
Has she got a mortgage? maybe a call to them may help clarify her position re: the lease