How bad can a character's past be before they're a sue?

I was wondering if there was a limit. See I'm role playing, and one of the guys is pretty good. His character is for the most part realistic. Now if he would've mentioned his traumatic past once I wouldn't be asking this. Apparently his OC witnessed his parents murdered, and his mother raped before she got murdered at a young age. Thing is his OC brings up it up all the time, but in real life, having known people who've gone through traumatic experiences, they usually don't talk about it. His oc does it like it's some contest to see who had the most ****** up childhood. So is his character a sue or stu whatever the boy one is called?