Your question is unclear. Are you asking what security configuration is necessary in order for a user to reassign work which is not assigned to him ? What is the relationship between the manager and the original and proposed assignees ? Are they all in the same work group ?
The Perform privilege is commonly used by managers to access and reassign all worklist assignments. This privilege is granted with the PegaRULES:WorkMgr4 access role. It would be worthwhile for you to review the AccessWhen rule (Assign-Worklist.canPerform) to see why this grants permission.