The first thing you should do is to extract all the posts from that user to a dedicated table and then take your time to perform some thorough checks.
Check whether the post_id's are still available (join with the current posts_table: if there are no records so much the better, if there are you'll have to edit)
Check whether the topic still exists (you may have deleted a topic if no other user posted in that topic, in which case the topic has to be inserted also).
Just check everything you can think of (e.g. does the forum in which each post was made still exist).
At the end you'll probably want to set the user's postcount correct also, (and the postcounts per forum, per topic).
It's feasible but not something you should try on a live forum before having tested the procedure.
IIRC there was a similar request about 2 months ago (for a lot more posts even).
Spelling is freeware, which means you can use it for free.
On the other hand, it is not open source, which means you cannot change it or publish it in a modified form.