@sally1968,
Go talk to a lawyer. Make sure they are familiar with laws associated with hacking and unauthorized computer access.
In the US, you could likely be charged under the Computer Fraud and Abuse Act. The fact that she gave you the password does not mean that you continue to be authorized to access the account.
I'm not sure what the laws are in the UK, though.
Even if you forward the information anonymously to those affected, it's possible that the data breach can be traced back to you, especially if you have accessed the mail account via your personal device(s).
Be careful. Don't get a criminal conviction.