I know that even Liam doesn't speak in detail of what happened but it wasn't just sexual assault, it was a very vicious violent and frenzied attack. Liam nearly didn't survive. But it happened so quickly and it was a large nightclub, by the time Beau realised Liam was gone longer than he should be, it would've been all over. Guilt is understandable and it definitely contributes to Beau's struggles with his mental health but it's something he's always struggled to talk about. I'd say, give him the option that he can talk to you about it if he ever needs but don't take it personally if he doesn't. It took Lachie a very long time to be able to talk at any length about his shooting. Eventually, he opened up and it got easier the more he did but even to this day, he prefers to leave it in the past. The scars are reminder enough. Just keep reminding Beau the lines of communication are always open. We're human, we can so easily forget that.