PDOException on Article page : Base table or view not found
As per @Triskelion comment:

When you created the 'tags' field, two tables (field_data_field_tags and field_revision_field_tags) were created to hold the information. If the first table is present but the second is not, use a statement like CREATE TABLE field_revision_field_tags AS SELECT * FROM field_data_field_tags; to replace the missing table. If both are missing, then you should restore your database from your latest backup.

If the error is showing, the field has already been created and the database is corrupt. If he tries to recreate the field before trying to recover, he might lose all existing information. Better to begin with the restoration attempt, then move to more drastic measures if restoration is not possible.