One really handy thing with the Steam Deck is the ability to remap all of the buttons (as well as the two paddles on the back for each hand), so one could probably make a decent one-handed control setup for 99% of turn-based games. Even ones that require the use of the mouse, given the Deck’s touchpads.
Vampire Survivors requires nothing except the stick/dpad outside of menus (and I’m pretty sure you can use the touchscreen for menus, too).
If your friend(s) are stuck using the dpad, it might not be suitable, but Crypt of the Necrodancer only requires four buttons or left, right, up, and down (and you can assign buttons for the button combos normally required to do things like use bombs). This assumes that they like rhythm games.