PBM,
I have to admit I have more questions than answers. They both had to endorse the check, no doubt about it. You ask if it matters that it went to pay a marital obligation, my question is how do you know it was a marital obligation? You say a cashier's check was made payable for rent, do you know for a fact they lived together at that time, that the individual was the landlord?
You want to know if she only owes half the amount of the check. Yes, provided the check really did go to pay for a debt incurred in the interest of the marriage or the family, but how do you know that?
Since you have a signed affidavit, I believe you need to collect the full amount of the check, then get the funds split properly between the two of them.
If you haven't reviewed it already, state statute 766 covers marital propery.
Good luck on this one, let me know how it goes.
By the way, PBM, we have a common friend - Face.
Opinions are my own and not those of my employer.