Dealing with a frustrating player

I have been a part of a Dungeons & Dragons campaign for almost a year now (I am not the DM) and we have a player who has, within the past few months, quickly become very difficult to deal with.

  1. He is notoriously late, often an hour to an hour and a half regardless if we met online or in person. When he finally showed up, he would get on his phone to watch unrelated videos with the volume up loud, carry on conversations in the middle of a battle, and not pay attention until it was his turn. When it was his turn he would ask "alright, so what’s happening?" and proceed to reiterate everything that happened while he wasn’t paying attention. If it wasn’t his turn, he would attempt to force the story back onto him.

  2. While we met online, we noticed was that this player suddenly starting rolling really high for someone in a single digit level and succeeding in everything that he wanted his character to do. When we met back in person, he started hiding his dice behind a "plastic bag fortress" and grabbing the dice before we could confirm the number and call out "Nat 20!" (which in our campaign is an automatic success and less than a 5% chance of rolling it per game, let alone an individual session). This made the rest of us annoyed because we couldn’t prove it and obviously couldn’t do anything against him. (but come on, players don’t need to hide their dice!)

  3. His character is a gunslinger and therefore, before the rest of us got a chance to do anything, he would take out his gun and shoot anyone who disagreed with him, regardless if it was an NPC or another player. And his alignment would change with whatever he felt his character needed to do in that moment. This would naturally lead into sometimes an unwanted initiative where he would be unstoppable thanks to his suspiciously high rolls.

  4. Our DM’s younger sister joined us for a brief period with two young (female) characters, one a rogue, the other a sorcerer. This player decided that his character would take them "under his wing". In other words, tell the rogue she wasn’t allowed to steal from people and tell the sorcerer to train to use a sword and not wield her magic. He went after multiple female NPCs (and a couple of player’s female characters) to try and force in-game relationships that (while fun) were quite unnecessary.

  5. Despite his high rolls and ability to succeed, he constantly asked where his initiative modifier was located on his sheet, he didn’t know which one was the six-sided dice, and refused to call characters by their real name (because he didn’t try to remember them). He claims that the out-of-game arguments are "playful banter". Also, that he has been under a lot of stress (which is no excuse to cheat!). Everyone in our campaign uses D&D to de-stress and he takes the fun out of playing and ends up causing more stress to the evening. He has even caused two members of our group to quit.

Our DM has put up with his antics for a lot longer than he should have. While these are obvious red flags, he wasn’t like this at the start of the campaign. It felt like once we started meeting online he took it as an opportunity to mess with us and it carried over when we met back in person. We don’t know if he understands how much stress he is causing or is simply ignoring it in order to feel better about himself. We know that if we do confront him he will oppose the charges, or, if he does change, will only revert back when we become comfortable in our game-play.

We can’t deal with his cheating, manipulative, disruptive behavior any more. How do we gently (or not-so-gently) tell him that he straight up needs to leave?