Rules Question

In my last 2 games I had a player called Mid or ill troll on my team. Both games, i got called mid first, they acknowledged it and then in the second game they got super toxic and trolled. The first game the picked illoai support and played fine so i was like, well i can forgive your flaming cuz u tried ur best but then in the second game i went akali and they went miss fortune. They just went mid with me and fed. I have two questions, first, how do i report them if i didnt do it right after the game and two, they claimed they were allowed to do it because they were just "changing the meta" but i'm pretty sure that stealing someone's lane and then flaming them cuz they did badly as akali support isn't "changing the meta" at all. what is riot's ruling here? I believe they called for my report too. This is my friends account btw, im just using it to ask this question so i dont have to disclose my username. If u have any questions just ask me.