A married person enjoys a slightly higher SRCOP than a single-parent (€45400 v €40400), hence pays a lower amount of tax than the single parent.
This amounts to a 'married advantage' of €1050 per annum (€5000 @ (41%-20%)). Additionally, as DB74 noted, you'll also save the rent of the flat, so that would be added to the €1050.
If the person in receipt of disability would definitely lose that benefit then the answer is going to depend on how much the rent saving is. You can then compare the savings above v the value of lost disability income.
I have uploaded an Excel 2007 tax calculator [currently in beta and it won't work in Excel 2003] which might help you evaluate this query (but
'caveat emptor'- it mightn't give the 100% correct answer so if in doubt, get professional advice). If interested, it can be downloaded here:
http://taxcalc.eu/monthlyss/dlf/beta calculator.htm