When your mii or Natsuki said "Things aren't great with me or (Your Mii or Natsuki)", there's a "Move on" option that could break them up. Also, did you check my reply on my post? I saw your reply.

break something else iykyk

use a save editor to lower the relationship or to make em exes

When ever you boot up the game the friendship with miis has a chance to go up or down. If you want you could try reseting to get them to dislike each other. It will only change the first time you boot the game up. So you have to wait between sessions.

yeah, morgan, i saw your reply, i added you also everyone else, i tried to edit my save file but it corrupted my save and i had to revert back to my backup save

Usually what i do to break up a couple is save scumming (close and reopen game for different results) until i notice that the couple is in a fight. Then after that save progress and wait until the next day for the ¨i want to break up with __" news.