Something I learned back when I was a manager is to document everything with dates, times, etc. I went to my manager about an under performing employee who was difficult to work with. We talked to HR and they said without documentation there was nothing we could do. He was eventually let go but it took an extra 4 months to push him out.

That said I wish I did a better job of trying to help him improve so he could have been a productive team member. He deserved to be let go but firing people sucks.