QDOT info
While I have dealt with a number of similar matters, your question would be difficult for anyone to answer without knowing a lot more, such as: Is the husband resident in the US? What country is he from? Is there an estate tax treaty between the US and that nation? Do the couple have assets outside of the US? More importantly, one would need to see the trust documents themselves.
Your question makes clear you know what an "AB" trust is. A QDOT trust does not often have the typical AB trust set up where everything over the applicable exempt amount goes to the A (or "Marital Trust") and everything up to the applicable exempt amount goes to the B (or "Credit Shelter" Trust).
Because the applicable exempt amount for foreign nationals (save with regard to relevant tax treaties) is limited to $60,000, there is often not much point in even sheltering such a small amount in the B trust. Consider how long would a spouse survive on the income off of $60,000. I prepare QDOT trusts for my non US citizen clients but, on their instructions, do not put in the B trust.
From your question, it is likely that if the husband dies first with the wife surviving him by at least 30 days (or any similar period set out in the trust), then anything left to her -- as she is the US citizen -- is free of US estate taxes further to the marital exemption. There may have been a credit shelter trust for the applicable credit amount of $60,000 (or the amount in any relevant tax treaty) but it depends on the drafting If the US citizen wife died first,and the husband survived her for the applicable period, then she could leave up to the applicable US citizen exempt amount to him, anything above that would need to be in a QDOT for the estate tax to be deferred.
With regard to tax treaties, these may in some cases, such as the estate tax treaty with the UK, allow the same exempt amount as US citizens, but only where the UK citizen's worldwide estate is reported and taxable. For this reason, UK estate would often choose to be taxed only on the US situs assets and take only the $60,000 exempt amount.
Kevin Burke