Make sure you don't allow editing thread title beyond a short time period.
This is a basic instruction of what you need to do.
Create a test forum with your most recent backup, one that contains the original thread titles before your member made the edits. Open two tabs each with phpMyAdmin, open the xf_thread table in the backup database in one tab and xf_thread table in the live database in the other tab, copy title from backup to live for each thread that was edited.
I left out the part where you find the modified thread titles for the user, again you use phpMyAdmin for this.