Need advice about an under performing employee

I work in a leadership position at a warehouse in Inventory Control. I'm in charge of a small team of counters. Lately, one employee has been under performing. For instance, as a final counter, they should only have 5 errors in one week. Last week, they had 20 errors and this week they have 29 errors. I just found out recently that they are moving to another state and I believe this may be why they are getting so many errors. I think that they may feel as though they don't need to perform well since they will be leaving the company soon.

I've had to talk to this employee about their performance before, however, they blamed me for their errors. They said it was because I was making them only count first counts. Since then, I have tried to make sure that they get a chance to count second and final counts and their performance did improve for about a month.

Now, though, they're back to getting lots of errors. I know I need to talk to them about their performance, but I'm honestly not sure what to say to them. That's where I need advice. How do I go about handling this situation? I can't talk to my manager about it beforehand since I won't see him until after I have to talk to this employee. I just want to make sure I handle this in the best way possible. Thank you.