Well, Butt, I think the lesson here is to never sign in to Gchat. Not only is the boss reading that shit, so is the NSA, the CIA, Alex Jones, and possibly even God herself – assuming that She isn’t otherwise engaged in something holy like protesting against gay marriage or protecting America’s borders from starving, abused child immigrants.
I’m not saying Gchat is for total chumps (I mean, we already have Facebook Messenger for them, AMIRITE?), I’m just saying that it shouldn’t be used for anything other than sending idiotic emojis and links to videos featuring adorable pets. If you’re not passing on a link to a Chihuahua dancing the merengue or a kitten climbing up a hipster’s skinny jeans, stay the fuck off that shit or you’ll end up in Gitmo with a pair of jumper cables clamped on your testicles.
However, since you already went down that wormhole (I’m assuming your friend knows that you read his missent chat), my advice to you is to not make assumptions that exceed the information you have available. Yes, he may have referred to you as a sniveling, whiny little soulless fuck, but perhaps it was 8:30am and you drank the last of the coffee.
There is also the possibility that you actually are a sniveling, whiny little soulless fuck – or, maybe his shit-talking was of a much more innocuous nature. Perhaps he insinuated you had a bug up your ass. Regardless of the severity of the insult, this important information shouldn’t be disregarded. I am not saying you should use it to stoke the embers of your anger, but rather that it could be a data point that bears further exploration. You might even ask him about it once you’ve simmered down. He is surely expecting some inquiry and like you said yourself, he is your friend.
Keep in mind also that while you may feel insulted by the actual statement(s), you are probably just as upset that he was talking about you – negatively – behind your back. Recognize that, but recognize also that nearly everyone just short of Jesus has done the same thing at one time or another.
Lastly, if you choose to play the victim here you will be victimized – actually, you already have been, so cut that shit out. Laugh it off, and realize that you’re not the nearly flawless person you believe other people think you are.