Hi Steve,
Deleting a tag should not delete the work units/hours themselves. It only removes that tag from the entries. Most likely the entries are currently hidden because the list/report is still filtered by that tag, or by a date/task/search filter. Please try clearing all filters in the timeline. The entries should then appear again, just without the deleted tag assigned.
If the work units were actually deleted separately, recovery is only possible from an existing WorkingHours backup or cloud sync data. If sync was not enabled and no manual backup was created, WorkingHours would not have another copy of that local iOS data. The paid version/sync on your device would not contain your wife’s local data unless her device had sync enabled to the same sync file/account.