There’s a workaround: play the game until you are level 30.
Multiplayer has fewer quests than singleplayer, and you cannot reload your game when you're killed and instead need to respawn in town and then return to where you died to recover your equipment. Goat Shrines and Cauldrons can also permanently gimp your character, so you basically can't touch them in MP without the ability to reload. If you aren't actually playing with other players, you really don't want to be playing multiplayer.
Grinding to level 30 also takes a ridiculous amount of time, especially if you get bad loot drops. My Sorcerer just couldn't find a tome of fireball for love or money, and that meant I was stuck grinding levels 5-8 nightmare, which is way, way
slower than grinding levels 13-15 (but without fireball you just aren't going to be able to kill stuff consistently on those levels).
(But seriously, what is your use-case here? It does not seem fun to me to get a second hand character with someone else’s taint all over it)
He's trying to use a bug in the game. If you start a multiplayer game in hell difficulty, return back to the main menu, and then start a new game in singleplayer, then the difficulty of your singleplayer game will be hell difficulty. You do need to repeat the process every time you restart Diablo, otherwise it will revert to normal difficulty the next time you visit a dungeon floor you haven't been to yet.
The character itself is completely irrelevant since he won't be playing it, he just needs it to be level 30 so it can create hell-difficulty games.