With the release of patch 8.10, Fortnite has done some changes to the way its cross-play support actually works.
First of all, Epic Games combined Xbox One and PS4 pools. This will require you to opt-in to cross-play, so it’ll only work if you want to.
Surprisingly, Nintendo Switch was excluded by this pool and joined instead the pool of the mobile platforms.
“Before Switch players were combined with Xbox One and PS4 cross-play parties,” said Epic. “We expect an on-average better per-game experience for both Mobile and Switch players.”
“Motivating factor is unlocking optimization potential allowing us to run more playlists during more hours of the day while supporting more data center locations.”
Again, the move was quite surprising since Nintendo Switch is such a big platform right now and players would’ve probably wanted to keep playing with them despite of the version.
So, if you have a friend on Switch, the only way you can play with him moving forward is that you play on mobile or on Switch itself.