Should I save the character or not?

I'm planning on writing a Nuevillette x Demon slayer crossover because I can't find any. And I'm planning to introduce Nuevillette to the other Demon Slayer during the Mugen train arc.

Should I write that Nuevillette saves Rengoku or let him come a bit late and observe as he isn't completely familiar with everything yet, and not able to save Rengoku?