Wow!  Almost seems like the manager should be getting a poor performance rating for failing to take care of Juan here.

From an onboarding perspective, I've often found the best way to deal with performance issues is to set the stage on goals and expectations early in the job.  This way, when the performance takes a dip, you can both go back to the expectations of the job and see where they are falling short.

From a performance management perspective, my personal philosophy is that performance management is a year-round activity.  Having discussions about performance throughout the year is critical to staying on top of these performance issue.  It also gives the manager the ability to tweak the expectations as the bussiness environment changes.

Staying on top of goals and expectations, documenting them, getting agreement and having regular dialog can helps take some of the emotion out of the discussion.  (seems like this story is full of emotion).  The more fact-based, the better.