Returning player with a beginner friend

I've played the base game to completion and have an old character at powerlevel 17.

When I played, there was no way to reduce powerlevel/to play with a friend and not impact their world level.

A friend just got the game and we're planning on playing through together and I'd really like to avoid creating a new character if possible, but I'm not sure if playing my old character would "ruin" the game if I join a new player?

I know there was a cap on how much another player can impact your world level, but idk how much that realistically achieves.