Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Sorry for your loss.
If this subscription was purchased from Best Buy, billing matters will need to process via them. See Microsoft's explanation here: https://support.microsoft.com/en-us/accounts-billing/subscriptions/manage-your-microsoft-365-subscription-purchased-through-a-third-party
Mind you, Best Buy often tells their customers to contact Microsoft. You might want to share this article with them if they do that.
However, if the subscription was signed up directly with Microsoft, do you have access to her account? If so, go to https://support.microsoft.com/contactus, Type "Billing" and press Enter. On the next screen, click "Contact support" at the bottom of the page, then choose "Microsoft 365" > "Manage my subscription". You’ll then see the option to chat with a Microsoft support agent.