If you’re both on Amazon Prime or Apple whatever their version of that is. you can share playlists. I think that’s the closest (legally), though it requires them already having prime or whatever, so it’s not a financial gift.
Unless there is some reason it has to look like a tape cassette, like if he were using an actual cassette, I would walk into the closest store where they sell USB drives and ask for the cheapest (smallest) one, which will surely be several gigabytes at least, but who knows what they have left behind the counter?
As for the music, to have it “play on any platform” I guess mp3 is still standard and popular; alternatives are FLAC and AAC.
That made me think that somebody should have made a fully self-contained MP3 player that functioned like those plug-in cassette adapters there used to be for CD players. And it looks like someone tried–I don’t know if it was ever actually sold, though.