In this situation, you can use the in-game mobile function if you've seen their name and message them stating what has happened, however bear in mind it's not their fault that someone else has supposedly broken rules... you can pass a note when in the roleplay scenario stating your OOC problem, however preferably leave it till last when the situation itself has stopped/ you aren't being interacted with (might be in the naughty corner or whatever) Otherwsie, as previously said you should let the situation unfold as any other, even if something unfair* has happened.
*bear in mind you could be misinformed/have misconceived the situation and they may not have done anything wrong. It's a common thing that does happen, but if you were to go onto teamspeak or whatever mid situation, it completely ruins it!