What can my soldier do to prove he's paying his mother (dependent) in order to get his BAH back?

Tell him to stop paying her in cash. I assume this is for "rent"
Get a signed lease agreement NOW and a signed notarized affidavit from the mother stating she has received $$$ every month since the date the agreement was reached.

Open a Support-V allotment. (V=Voluntary)

Unless there is a lease agreement and receipts given to said Soldier for rent....he may very well not get BAH back and may have to pay back said BAH

If the Soldier's mother has been established as a dependant where is;
1- the approved DD Form 137-3 from DFAS
2- DD form 1172 ( DEERS)
3- counseling from Cdr
Immediately stop paying cash set up an allotment and get a notarized lease agreement.
Cash leaves no paper trail and is hard to prove. Unless he has an accountant who is keeping ledger recordings of assist, liabilities, cash in and cash out.

Without a paper trail, proof that that money has been being used to care for her, he's gonna lose it. They may be able to accept some written document from the person receiving the money (landlord etc.) that they are receiving X numbers of dollars every month from this person. It's unlikely but possible. The best thing to do is for him to immediately discontinue his cash method and switch to allotments. There will be a paper trail that shows every month $XX is being sent to her bank account and that should clear it all up.

Usually when an adult is carried as a dependent in DEERS, it is implied they are living with the SM. To get in DEERS they had to have been demonstrated as unable to care for themselves, requiring the SM care for them. This could be what DFAS is demanding proof over.

http://www.dfas.mil/militarymembers/SecondaryDependency/SDC/secondarydependency_Army.html

http://www.dfas.mil/militarymembers/SecondaryDependency/SDC.html

The soldier can set up a Totten Trust with her at a bank which issues to ATM cards for that account. The soldier makes the allotment from his mid month pay, and when she takes it out there is an automatic way of tracking it to her. That is absolute proof of the soldiers payment to her.

Have him write a check to start with. Also he will need copies of bills to prove what he is giving her is at least 50% of her expenses. Just giving her money isn't enough to claim her as dependent. He has to be paying at least 50% of her living expenses

Easily proved to DFAS by something as simple as setting up an allotment and not using cash.

Guarantee There is no paper trail for a reason. I bet the mother is on some type of welfare And a paper trail will reduce it.