A week after posting the original question, I am amazed at all the responses. I really want to thank each of you for the time you have taken to respond. I have shared this with the wife, and she is amazed and thankful as well. Let me try to answer a few questions, provide a little more information, and give an update.
How long the parties had been married when the Participant died? 3+ years
Was there an antenuptial agreement, or a postnuptial agreement, or a marital settlement agreement, or a Joint Last Will and Testament, wherein waiver language might be found? No, no, no, and the husband that died had no Will.
Did Fidelity provide notice to anyone of the application by the son to receive the 401(k) account proceeds? No
The husband and wife lived in Tennessee. The wife still lives in Tennessee. Both the husband and wife worked in Alabama for different companies at the time of his death. They both received their health insurance from their own employer, so it does not appear there would necessarily be any way for the the husband's employer to have known he was married.
The company is a large corporation with over 200 facilities around the world. Their market cap exceeds $3.6B with annual revenue topping $2.4B.
As I stated in a previous post, the husband died in May 2019. The day after the funeral, the wife along with her sister sat down and contacted the company's HR department as well as Fidelity about her husband's death. Once informed, it appears either HR or Fidelity started a process. She was told the designated beneficiary would receive a claim form. Since the son was designated as the beneficiary, he received it.
Since the husband did not have a Will, his estate was subject to probate. Three people had claim to his estate: his wife, his son, and his 2yr old adopted son. The probate lawyer representing the wife dismissed the possibility of the wife being entitled to the 401K since the son was designated as the beneficiary. As a result, the wife dropped the pursuit of the 401K.
In order to make the claim to the 401K, I am certain the son had to provide a death certificate along with the claim form. The death certificate lists the husband as having a wife at the time of his death.
As suggested in the first reply in this thread, the employer's HR was contacted this week and asked to provide a SPD. They either couldn't or wouldn't. They were also informed the wife would be making a claim - was there a claim form that needed to be used. At this point the wife was told she needed to contact Fidelity.
The wife called Fidelity as well. Fidelity was told the wife was the executor of the estate and was willing to provide documentation. Fidelity was not helpful and once again told her she would have to submit a subpoena in order to gain access to the account.
At this point her probate lawyer has said he will send Fidelity a letter. He has also offered to refer her to a lawyer more specialized in this type of matter. As much as I had hoped this matter could be resolved more easily, it has become clear based on your comments and the responses she has received that she will need a lawyer.
The best part about all of this is that where there was no hope there is now hope. I wanted to see this as just a big mistake that needed to be resolved. Since there is probably significant money involved, it is clear it won't be an easy resolution. I am surprised there does not appear to be any precedent on this issue. In my opinion, since either HR, Fidelity, or both received the death certificate clearly stating the deceased had a wife AND there was no waiver on file that allowed the son to be paid instead of the wife, mistakes were made.
Once again, thanks for all your help.