As stated by others, their Steam wishlist is always a good starting place.

If they like story games, To The Moon is like a book you wish you could forget so you could read it again for the first time. It’s best played with 0 spoilers.

If they are the type of person who tries to optimise everything and automates a lot on Minecraft, Factorio (a bit expensive) might take over their entire life for a few years.

If they enjoy puzzles and/or you want to play with them, Portal 2 has a co-op section.

I will happily agree with everyone who mentioned Stardew Valley. It can be single or multiplayer, and has a large number of mods. The Expanded and Ridgeside mods add a huge amount of content including trans characters.

Depending on how well you know them, you could try asking if there’s any game they’d like to play together that they don’t have. Having a friend to play with is often better than the game itself.
PULSAR: Lost Colony and Void Crew are mediocre games made fun by forcing interaction with other players (friends or random people in open games). They also have a surprising proportion of LGBTQ+ players.