Can you prep a spell outside your main phase in Aeon's End?
Focusing a breach and prepping a spell to a breach are separate actions.

Rules p. 3:


  "The only time that spells can be prepped to a closed breach is during the player's turn in which that breach was focused."


So when an ally focuses your breach, that doesn't help your prepping, because the focus didn't happen on your turn. (But it does make future opening cheaper. And it did happen on their turn, so if they had the ability to prep a spell to your breach that could work.)

When a spell you cast focuses your breach, that did happen on your turn, so you can prep a spell to it that turn (in the main phase) without focusing it again.