Open up phpMyAdmin
Go into pp_notify
sort by the column on the right, it's full of either 0's or 1's.
1's are the people that subscribed to categories. In there, you will see his entry.
On my site, there were very few people that subscribed to categories, most are for photo comment notifications. So it would be easy to spot on mine.. your mmv.
If you know his user ID (if you can actually recover what it was) you may be able to spot him just by sorting that proper field.
This should get you going mate, cheers.