If the account was in joint name, with right of survivorship, as they often are, she likely is now its owner.
If not, and it was in his name only, it is technically the property of his estate, and technically (in most states) she can't access it until the Will is offered for (with a chance of others to object) and admitted to probate and a distribution from the estate is authorized, after all creditors are paid.
BUT assuming that the estate owes no one money and she is the sole beneficiary, there would be no one to complain. As the Nike ads say, most people "Just Do It". That may not be strictly legal, but .....