opc.three (9/7/2012)
Taking a differential backup from the primary and restoring it to the secondary will re-establish your log chain, just remember to supply the proper option on the restore (STANDBY or NORECOVERY). After the differential is applied to the secondary your existing setup will continue per the usual LS backup, LS copy and LS restore job schedules.
That will only work if he didn't take any full backups since he started with Log Shipping!
