We had the same thing happen, tho DH was entitled to claim SS every year as long as he was current on CS as of 12/31. The year in question was when BM had CS switched from county enforcement to CSRU and there was a delay in CSRU getting the money to her, so she thought it was 'late.'
We claimed SS immediately.......I usually have our taxes filed within 5 days of receiving our W-2's, but shortly after that is when BM made the accusation. We provided her with everything, including a print-out from CSRU, but she wouldn't accept it. So we went to CSRU and asked for a notarized copy. By the time we got it and she finally accepted it, it was past April 15th, so we were certain she'd claimed SS, too.
Sure enough, right before Christmas that year, we received a notice from the IRS that a SSN had been used on two different returns and it was SS's. The letter said that if we'd filed our return properly, we could 'disregard' the notice. BUT......if we needed to correct our return, the form included needed to be filled out and returned to them. We knew we'd filed correctly, so we disregarded it. But after Jan. 1, I called the local IRS office and actually talked to a human. I explained the situation and asked what we could do to prevent this from happening again. Quote her: 'He who files first, gets.' Which is why our refund was not compromised that year and we haven't had a problem since. But we have no idea what happened to BM...I am assuming the IRS didn't give her the refund she was expecting, but she certainly hasn't given that information up willingly!
Long story short, if you file, BM will be notified (it might take a few months) by the IRS that your SD's SSN was entered on 2 separate returns. From there, it's anybody's guess as to the turn out, since in our case we filed correctly, but in yours, BM didn't. The problem is, it can take a long time to resolve and even if they determine that your DH is entitled to the deduction, who knows how long it will be until you receive it. They won't hold your entire refund, just what you're currently entitled to (w/o the deduction), then investigate and work out the rest.
If you still have questions, try contacting your local IRS office. You probably will have to weed through their phone bureaucracy but hold out until you actually reach a human. I'm sure they can give you more definitive information. Hope this helps!