I use this script to pull instances assigned to users: 

SELECT        [Process Name], [Instance name], [Started by], [Last updated], [Assigned to], [Assigned to (username)], [Assigned to (email)], [Current step], [Step start date], [Start date], [Instance ID]
FROM            (SELECT        instance.bp_name AS [Process Name], instance.title AS [Instance name], 
                                                    CASE WHEN start_user_snapshot.displayname LIKE '%WORKFLOW%' THEN 'Workflow' ELSE start_user_snapshot.displayname END AS [Started by], instance.lastacted_date AS [Last updated], 
                                                    CASE WHEN bp_worker_resume.status = 5 THEN assigned_user_snapshot.displayname WHEN bp_worker_resume.status = 2 THEN assigned_user_snapshot.displayname WHEN assigned_user_team_snapshot.displayname
                                                     IS NULL THEN teams.name WHEN bp_worker_resume.status = 1 THEN assigned_user_team_snapshot.displayname END AS [Assigned to], 
                                                    CASE WHEN bp_worker_resume.status = 5 THEN assigned_user_snapshot.username WHEN bp_worker_resume.status = 2 THEN assigned_user_snapshot.username WHEN assigned_user_team_snapshot.displayname
                                                     IS NULL THEN team_users.username WHEN bp_worker_resume.status = 1 THEN assigned_user_team_snapshot.username END AS [Assigned to (username)], 
                                                    CASE WHEN bp_worker_resume.status = 5 THEN assigned_user_snapshot.email WHEN bp_worker_resume.status = 2 THEN assigned_user_snapshot.email WHEN assigned_user_team_snapshot.displayname IS NULL
                                                     THEN team_users.email WHEN bp_worker_resume.status = 1 THEN assigned_user_team_snapshot.email END AS [Assigned to (email)], bp_worker_resume.step_name AS [Current step], 
                                                    bp_worker_resume.assign_date AS [Step start date], instance.start_date AS [Start date], instance.bp_instance_id AS [Instance ID]
                          FROM            dbo.cf_bp_main_instances AS instance LEFT OUTER JOIN
                                                    dbo.cf_user_snapshot AS start_user_snapshot ON start_user_snapshot.id = instance.user_snapshot_id LEFT OUTER JOIN
                                                    dbo.cf_bp_worker_instances AS bp_worker ON bp_worker.bp_instance_id = instance.bp_instance_id LEFT OUTER JOIN
                                                    dbo.cf_bp_worker_instnc_to_resume AS bp_worker_resume ON bp_worker_resume.worker_instance_id = bp_worker.instance_id LEFT OUTER JOIN
                                                    dbo.cf_bp_worker_instance_history AS bp_worker_history ON bp_worker_history.instance_id = bp_worker.instance_id AND bp_worker_history.status = 'assigned' AND bp_worker_resume.owner_snapshot_id IS NULL
                                                     LEFT OUTER JOIN
                                                    dbo.cf_user_snapshot AS assigned_user_snapshot ON assigned_user_snapshot.id = bp_worker_resume.owner_snapshot_id LEFT OUTER JOIN
                                                    dbo.cf_user_snapshot AS assigned_user_team_snapshot ON assigned_user_team_snapshot.id = bp_worker_history.target_snapshot_id LEFT OUTER JOIN
                                                    dbo.teams AS teams ON teams.id = bp_worker_resume.team_id AND bp_worker_resume.owner_snapshot_id IS NULL LEFT OUTER JOIN
                                                    dbo.team_members AS team_members ON team_members.team_id = teams.id AND team_members.member_rights = 3 LEFT OUTER JOIN
                                                    dbo.cf_users AS team_users ON team_users.user_id = team_members.user_group_id
                          WHERE        (instance.status = 1) AND (bp_worker_resume.status = 5 OR
                                                    bp_worker_resume.status = 1 OR
                                                    bp_worker_resume.status = 2) AND (bp_worker_resume.assign_date IS NOT NULL)) AS subquery
GROUP BY [Process Name], [Instance name], [Started by], [Last updated], [Assigned to], [Assigned to (username)], [Assigned to (email)], [Current step], [Step start date], [Start date], [Instance ID]
ORDER BY [Start date]

You can search by their old email or username to find the instance that is still assigned if it is active.