Money question about loved one who passed while on SSDI.

(Yes I will be calling on break to verify everything) My mother in law passed away August 15th she was on Disability. Her disability check hit her account August 20th and her partner took the money out. Disability came and took the money back out of the account (I'm assuming over drawing the account) Her partner is now saying that we need to send paperwork back to them because that check was realistically for the month prior and she is entitled to it. Is this true? If she was alive for half of August does that mean she is also entitled to half of august Payment as well? Has anyone ever dealt with this? If so how long did it take to get the check back?