Not for the faint of heart.... so backup your database twice and cross your fingers before doing this... but what you need to do is pick one usernumber of hers and delete it. Then you need to reassign everything from that usernumber, to their other usernumber. This requires updating a bunch of tables - I posted an extensive list of all the queries needed to do this in this thread:
http://www.threadsdev.com/forum/showflat.php?Cat=0&Board=php&Number=117468
