I have an odd request from management (is there any other kind?) and was hoping to get some info here.
Is there a way to mount an EFS volume from AWS account A to a windows EC2 instance in AWS account B?
Unfortunately you cannot.
From Authentication and Access Control for Amazon EFS
Cross-account administration – You can use an IAM role in your account to grant another AWS account permissions to administer your account’s Amazon EFS resources. For an example, see Tutorial: Delegate Access Across AWS Accounts Using IAM Roles in the IAM User Guide. Note that you can't mount Amazon EFS file systems from across VPCs or accounts. For more information, see Managing File System Network Accessibility