That's just a bug as well, but there's no decals or vegitation in that pic either which could be caused by the player alt/tabbing out of the game or your server is set to allow players to use /r_drawdecals 0
Conclusion, nothing bannable.
You can ask them to upgrade/downgrade his/her gfx/os drivers, turn off AA, but fact is.. you probably won't get it fixed. If I were you I wouldn't worry about it.