What you give up on this pairing
Being straightforward about it: no Mario Kart, no Smash Bros., no Mario Party, no Super Mario Bros. Wonder, no Nintendo Switch Sports. Those are Switch exclusives and there is no workaround - if they are what the Switch owner wants to play, the honest answer is that the PC player needs a Switch, not a different game. Our Switch page covers those on their own terms.
You also give up most of the big co-op shooters. Helldivers 2, Deep Rock Galactic, Borderlands 3, and Diablo IV are all absent from Switch or absent from its crossplay. Four-player co-op across Switch and PC essentially means Overcooked, Moving Out 2, Human: Fall Flat, or Terraria.
By what the two of you want
A shared world to build in. Minecraft first, Terraria as the cheaper 2D alternative that scales to eight players.
Chaos for a short evening. Overcooked! All You Can Eat, Moving Out 2, or Human: Fall Flat - all crossplay, all cheap, none requiring skill.
Free, right now. Fortnite, Rocket League, Fall Guys, or Among Us. Zero cost and zero risk while you work out what the two of you actually enjoy. See the free games hub.
Accounts and friend lists across the two
Crossplay games handle friending in one of two ways, and knowing which saves a lot of confusion. Most use their own account system - an Epic account for Fortnite and Fall Guys, a Microsoft account for Minecraft, a Rocket League account - and you add each other there rather than through Nintendo or Steam. Your Switch friend list is largely irrelevant to cross-platform play.
So the reliable pattern is: both of you create or sign in to the game's own account on your respective platforms, then add each other by that account name. If you have added each other on Nintendo Switch Online and still cannot see each other in the game, this is almost always the reason.
One more Switch-specific note: online play on Switch generally requires a Nintendo Switch Online subscription, including for free crossplay games. Fortnite has been an exception to that requirement, but do not assume it applies across the board - budget for the subscription on the Switch side.
Performance is not identical
Expect the Switch version of any cross-platform game to run at a lower resolution and frame rate than the PC one. For Minecraft, Terraria, or Overcooked this makes no practical difference. For anything competitive - Fortnite and Rocket League especially - the Switch player is at a genuine disadvantage against mouse-and-keyboard opponents. Play those casually rather than ranked, and nobody will care. Other combinations are covered on our PS5 + PC and PC + Xbox pages.
Docked and handheld also differ on Switch - handheld mode runs at a lower resolution still. For a long session of anything competitive, the Switch player should dock. For Minecraft, Terraria, or Stardew-style games it genuinely does not matter, and handheld is part of the appeal.