I work for a private company, and i have a manager who is several levels below the owners. He is usually indifferent towards most of my team, except for one person who he is very considerate and accommodating with. Recently i had brought to his attention that this person is underperforming and lacks knowledge in areas where he should be strong. He was the least bit concerned upon hearing this.

Last year, with another direct that was also underperforming on the same level, he was quick to respond with setting up a PIP.

I occasionally engage in conversation with the owners when they visit or by email.

Should i inform them of this unprofessional manager? 

I am also concerned that if the direct that was put on a PIP finds out that his peer was not put on a PIP, he can report discrimination.