Under the principles of cooperation with evil, formal cooperation participates directly in the intrinsically evil act of another person; material cooperation participates in the circumstances of the sinful act, but not in its evil moral object. From the moral standpoint, it is never licit to cooperate formally in evil. However, material cooperation is sometimes permissible.
Example: If you pay for the abortion of another person, that is formal cooperation. If you pay taxes and, contrary to your will, some of that money goes to pay for abortion or abortifacients, that is remote material cooperation.
See this article on when material cooperation is permissible.
For the case in question, helping to pay for an apartment for a cohabitating couple is illicit material cooperation; it is a sin.
There is nothing to necessitate your cooperation; you are doing no harm if you refrain from helping them. It is not like the case of refraining from paying taxes, which would cause you to end up in jail. Given that your help participates materially in their grave sin, and there is no compelling reason to tolerate this effect of your act, you cannot morally assist them.
I disagree with those posters above who say that the parish priest’s decision in the matter is authoritative. The teaching of the Church on morality takes precedent over a pastor’s judgment. If the priest had decided incorrectly, contrary to Church teaching, would we be obligated by his authority? Not at all. His judgment and authority carries weight because it is an expression of the truths taught by the magisterium.