[5e] That guy

I'm part of several campaigns as either a player or to shadow a dm to learn the rules and how to be a dm. There is a player in a few of the games I play or watch that is infamous in the group for coming late/no-showing/poaching magic items. The first two wouldn't be an issue if he told us he couldn't come or would be late, but he doesn't. The last part is where a lot of the group has an issue. He is known for making new characters and coming in with no magic items so he can take his pick of the loot. Again, not an issue except for the part where he's a DM and only shows up when there's a strong magic item to be gained because he knows when and where they are. For example, recently I was informed that he was no longer welcome at a campaign because he took a ring of magic holding from the pot as a druid. He was wildshaped the entire session and the rest of the table wanted the ring to go to someone who could actually use the ring in sessions. He refused and the rules were followed and he got the ring as the only caster with no magic items. People are starting to step up against him and I don't know how to handle this as a player or an assistant dm. The head dm has spoken to him about this but ultimately this is entirely up to the player's discretion. When so many people have also tried to reason with him and talk to him about it and gotten the same excuses, no one wants to play with him. The group's DMs has tolerated this much because we do not want to exclude him from campaigns but when all or most of the players do not like him and leave campaigns because of him, this goes from an issue among players to an issue among the group.
Edited for clarity.