Legal is a different matter.
As someone said, it's about a moral / ethical view.
The ex will still have to provide a house for the son - because he will be coming home from Uni during holiday time.
By providing a house, there will be costs with that, such as insurance, energy bills, mortgage etc - which will go down but won't halve.
The ex could sell the house and downsize if she can't afford the cost of providing a house that her DS can live in when he comes home.
She could find a way to increase her own income.
She could charge her DS rent when he is at home from Uni.
It's also complicated by the question of - is she supporting her DS at Uni as well?
When your DC goes to uni, your own housing costs will go down a bit but not significantly. In the UK , you are also expected to provide additional support to your DC at Uni as a parental contribution, if they don't get a full maintenance grant.