Is it legal for a bank to cash a check on a frozen account?

A court will decide if they are liable. This is a very "legal fact" specific question that no one can answer without reviewing teh entire case,a nd having the bank preent their defenses including why they thought they were obligated to honor the bearer paper. The probate court will will decide this issue.
It may turn out to be "harmless error"; so , if the presenter would have been owed the money by the probate courts decion anyway, then there was no harm to the estate, then there was no damage, a "wrong" must have a damage. If the debt would not have been honored, and teh probate court decides the check was not for a "legal debt"; the probate court could inlcude the bank as to debtor to the estate. If they violated federal or state banking law, the state or feds woulkd have jurisdiction over any "spankings " the bank may have coming. Thi issue has to be heard in court, otherwise, it is just guessing.

One reason banks "freeze" decedent's accounts is to keep the family from draining funds, especially since there are almost always valid checks that have yet to be presented for payment.

This often leaves widows with a financial challenge -- can't get at their own money -- and is why it is a good idea for each spouse to have a small account of their own.

Reader has a sound idea, there must be 10 questions about having bank accts seized or drained. No should ever put every penny they have in the bank, or in joint accounts. In my opinion, people shoudl have a least a week or even two weeks "neccessity money" in a fire proof home safe ( not under the mattress, cookie jar , or freezer) in a hidden location. I am all for people honoring their debts ( which is why I advise people not to ignore creditors suing them, yes they will get it anyway, but at court you may get to make payments that suit you raher than 1/3 of your net vanishing each week) ; but like the lady who was getting her SS settlement and her lawyer had a seizure in place thatwas going to leavce her peniless, or in the above situation, where spouse or children may need immediate money, or a question last month when someone left their ex spouse on the account and lo and behold, 6k vanished. "rainy day money" was called that for a very good reason.

The check that was cashed on the frozen account was written in someone else's handwriting to the grand daughter inlaw of the deceased with his apparent signature on it. I thought that with a frozen account no one could get access to the funds.

Does sound pretty fishy; I would certainly investigate ( the prbate court and /or the administrator.
Would there be any reason, such as a debt for the check? What does the gd-i-l say about it? If you really think there is some wrong doing, you can certainly talk to law enforcement about it. Was the date on the check recent? Could he have be corerced? or fooled ( while on medication) Have you talked to the bank? what is their explanantion? Perhpas someone called them , and told them to honor it? as I said previously, if they made a mistake, AND there was no debt to satisfy, they may have to "eat it" if the court makes them a debtor to the estate. Who froze the account? Maybe the account was not froze after all, or maybe it was froze on for checks subsequent to the death. Many questions to be answered.