People might find it necessary to complain about work when their work is physically unsafe, or they are being harassed or exploited, or they are not being fairly compensated for their work, or they are being discriminated against, or many other reasons. Complaining about work is so necessary that in the US, it’s a form of protected speech.

those friends aren't at work with you, so you have to explain the whole situation to them, and then, because they weren't there, they can't provide additional facts. you don't expect a listening device hidden in the shared company break room if it were a physical room, you just don't talk about the boss as if he's not there when he's actually there. similar expectations should apply to a digital space called a private channel when they're not there.