Hello, I am very sorry to hear about your friends late husband and this problem you have. I am afraid you will not like my answer but I have been helping people with yahoo accounts for years.
The way it works is this. Yahoo will only close the account, they will not give you access to it. All of the information is on this page:
That being said, you as an individual cannot do anything about this so I would not spend time looking for ways into the account. The only thing I could suggest is to have your friends solicitor get in touch with yahoos legal department and most likely they will need a court order for yahoo to let them into the account. I am not a solicitor I am an IT professional so I cannot speak much on the legal end of things.
What I can tell you straight away is you or your friend cannot access the account without the password. Yahoo will not let you into the account even if you provide a death certificate which would need to be provided by a family member and even then all they will do is shut the account down. All of this is for security reasons and yahoo is protecting the security of the account even though the owner of that account is deceased.
Read through the link I gave you from yahoo and will clearly explain what needs to be done to shut down the account but it will also explain that they will not give you access to the account for security reasons.
Because of this I again suggest you tell your friend to have a solicitor deal with this matter. As an individual there is nothing you can do I am sorry to say.
At best, ***** ***** this answer saves you a bit of time and I do hope it all gets sorted. Please take a moment to rate me by clicking the stars above, please click 3 or more which is how I am credited for my time and expertise. You can also reply back to me even after rating. The rating is meant for me personally, not yahoo, or their policies or procedures which I understand are very frustrating.