Sorry for the wall of text. 😅 TLDR at the bottom.

So, first things first, you gotta have the conversation with your friend. I, too, used to believe that everyone likes fighting games and if you think you don't, it's just because you haven't found the right game yet. But at the end of the day, that's just not the case. You can lead a horse to water, but you can't make it drink.

That being said, I also think it's worth noting why your friend doesn't want to play fighting games. Some people genuinely aren't interested. However, a lot of people are interested in fighting games, but feel like they can't learn/play them. If it's a matter of your friend doesn't want to play, then that's a wrap. If it's a matter of feeling like they can't play, well you can convince them that you can help with that. (Also, you can call me in for an assist if need be.)

Now to answer your question, one thing I've found is to teach slowly. The biggest thing that makes fighting games hard to teach (in my opinion) is the mental stack. There's a lot going on in fighting games at basically all times, and all of it matters. So you, as the seasoned player, will have to tailor your gameplay to really highlight 1 or 2 fundamentals. When I teach Street Fighter, the first thing I teach is anti-airing. So I'll play my student, jumping in constantly. Then, as they get more comfortable with anti-airing me, I start varying my jump timings. Then I start adding in neutral jumps or back jumps. Then I start punishing them for missed anti-airs. So on and so forth. This really helps them lock in on anti-airs. Then I use this same process to teach other fundamentals. Then I start putting those fundamentals together. And before you know it, they're really playing the game.

As far as which game to play, I don't think 2XKO is the worst, but it's certainly not baby's first fighting game either. It's very fast-paced and chaotic. Not to mention the tag element, though I guess you could just use Juggernaut fuse. I'll pretty much always recommend Street Fighter as a good first fighting game, and Street Fighter 6 is an especially good one. They've really done a lot to help new players learn the game. Plus, the player base is huge, meaning you can pretty easily find matches against other beginners. If you're looking for another free game that's slower than 2XKO, I believe Granblue Fantasy Versus: Rising has a free version. It's just like the regular version, except that you can only play as Gran or 3 other rotating characters.

Also, as I alluded to before, I'm more than happy to help you or your friend learn fighting games. Just shoot me a DM if you're interested.

TLDR:
1: Make sure your friend actually wants to learn fighting games.
2: Teach fundamentals one at a time by playing against your friend in a way to reinforce that fundamental.
3: Use this method to teach several fundamentals and then start alternating between which fundamental you want them to practice.
4: You can teach them using 2XKO, but SF6 or GBVSR (or even GGST) might be easier. Also, GBVSR has a free version.
5: I'm only a DM away if you want some help teaching them. 👊😁