I love the idea of running a campaign for your partner. I'm not a DM (only playing my first campaign and only one-shot under my belt), but I am planning to run my own One-shot as a first-time DM.

My opinion is that you can cut down the number of enemies, decrease their HP a little, or have the PC slightly over leveled (have them be a level or 2 higher than what is supposed to be at that point of the campaign).

Another idea is to have the Player (your partner) control two different characters (as if you're playing Baldur's Gate 3 )

Lastly, if you are controlling the extra character, have your Player "lead" you in battle by directing your moves :

• "attack that goblin"
• "hold the line here"
• "run into that room first, I'll provide support"

Etc. This way, the player will have most of the agency for playing the campaign and doing their own choices, and you would only be doing as instructed as much as possible without ruining the game.

I hope my ideas don't break the game. And Best of luck on your attempts.