Most MFA have backup codes that you can download during first setup. This is typical for the MFA using QR codes. For things like banks where they email/text you your temporary code, many already have processes in place for when someone passes away (ie typically someone needs to contact the bank with proof that xyz passed away).
You can just save and print out the QR code onto paper and put that in your lockbox. Then your bequeathed can just scan those with their MFA device. Be sure to label them ;)
