Is a person allowed to submit a complaint about an employee that is behaving inappropriately to customers, or is it considered Lashon Hara?
There are multiple cases where a person is allowed to speak against somebody else when it is לשון הרע לתועלת, which means there has to be a very good reason to complain about somebody else. The prohibition of Lashon Hara contains two central prohibitions. The first is to speak against your fellow Jew, which is why even if it is true, it is forbidden, because saying negative things about a Jew is forbidden.
The second prohibition is to say anything that would cause the person damage or embarrassment. The Chafetz Chaim based on the Gemara and Poskim says that we have to follow seven basic conditions in order to speak against a fellow Jew:
A person needs to have witnessed the actual event, not having heard it from somebody else.
Besides witnessing the event, he needs to investigate, learn the details of the situation, and be convinced that the person behaved inappropriately.
He needs to try his best to fix the problem before telling anyone else, like the Shulchan Aruch says in Chosen Mishpat Siman 421 Se'if 13. If a Jew sees another Jew hit someone, if he doesn't have any other option, the observer should hit the aggressor to solve the problem.
You must rebuke the Jew first before telling anyone, and you should rebuke him nicely and calmly. Perhaps by rebuking him there will be no need to tell anyone about his behavior because he will have stopped.
Once there is no other option but to speak with other people about his behavior, you must be very accurate discussing his behavior and not exaggerate his actions. The source for this condition comes from Gemara ערכין דף ט"ו.
When telling other people about the event, you should be telling them with pure intentions and not because you want revenge, and it should not be accompanied by any joy or pleasure. The source for this condition is the Chazal who tells us that even when Hashem sends the goyim to punish us, they will be punished because they did it with joy. We learn from this that even if you are doing the right thing, you should do it because you are forced to and not because you want to.
The Gemara in :שבועות ל says that we are not allowed to cause additional damage to another Jew besides for what he deserves based on Halacha. Therefore he needs to make sure that this individual will be punished financially, but not if it will lead to a great financial loss that he doesn't deserve. One should consult a rabbi.
In conclusion, since it is very difficult to fulfill these seven conditions, and some hold that there is a biblical prohibition, one should always consult with an objective rabbi to know how to properly proceed.