According to R. Menahem HaMeiri the definition of l'shon hara, is speech that is meant to be defamatory or hurtful. Thus, statements of a negative nature that are said for a specific reason (in this case providing useful feedback to one's employer), and not in order to hurt others, are permissible since by definition they are not l'shon hara. These are his words in is Hibbur HaTeshuva (Meshiv Nefesh I:4):
עקר לשון הרע הוא המתמיד לספר בגנות האחרים ויתן דפי בענינים או שילעג להם למעוט הצלחתם...ומעין זה מי שילעיג על חבריו למעוט השגתם בידיעה...לכונת לעג וליצנות...וכלל הדברים כל אשר יתן מום באדם...ללא כונת ענין נבחר הוא ענין לשון הרע...שאין לספר במומי בני האדם על דרך לעג וליצנות, אם לא יעשה כן...לאיזו כונה
The archetypal l'shon hara is one who frequently relates embarrassing things about others, and adds falsehood to what he says, or he defames them on their lack of success...And similarly one who defames his fellows on their lack of intellect for the purpose of defamation and mockery...The rule is that anyone who ascribes a fault to others...without some specific intent is l'shon hara...For one should not relate the faults of others in context of defamation and mockery, unless he is speaking for some purpose. (Translation my own).
This also appears to be the opinion of this is also the opinion of R. Elhanan Wasserman, who generalises this to all interpersonal mitsvot. He writes in Kovets Ha’arot (Yevamot: 70):
ולולי דבריהם היה נראה, דכל האיסורין שבין אדם לחבירו אינן איסורין אלא דרך קילקול והשחתה שלא לצורך...
וכן בלאו דלא תלך רכיל, מותר לספר לה"ר על בעלי מחלוקת כדי להשקיט המריבה. וכן בלאו דאונאת דברים...
ומוכח מכ"ז, דכל האיסורין האלו הותרו לצורך תועלת
If not for their words, it would appear that all interpersonal prohibitions, are only forbidden in the context of degrading and tarnishing others for no purpose...And so too with the prohibition of gossiping, it is permitted to tell l'shon hara on disputants to quell the dispute...And similarly with the prohibition of onaat d'varim [verbal abuse]...And it is evident from all this that all of these prohibition are permitted to achieve an end. (Trans. my own).
Thus even with a live person it would be permissible.
Importantly, they do not attach conditions to to'elet, as the Haffets Hayyim famously does.