Well, just so you know, there are a lot of Mary Sue's Gary Stues in the Role Playing world. People just like the idea of a better than they are person, and for some reason, they just go to the extremes. Even if you make their image perfection and their history normal as any other persons, their still considered a Marry Sue. Even if you give them your middle name, they are considered a Mary Sue.
So I find it a bit funny that someone is asking for help in telling their friend their character is a Mary Sue, but that may just be me.
I have one of those characters on another site. But she had a hurtful past, so it's a bit easy to tell why she's the way she is now, and her job helps out with it. It's a medieval role play. She's a courtesan - a person who gives her self to the powerful. But she's not exactly a courtesan, she's also a female Robin Hood. Reason why? Her parents sold her off to rapists. All she knows is lust, but she wants to do it for the greater good. She finds out that there's some supreme being with a horrible secret or plan going on, and she goes off and tries to get them to think of something for the greater good. I don't actually know about that part yet because I haven't rped that yet, but I'm plotting with one of my friends to get that going.
Now, if the character seems confusing by finding out about it's history, you should write and outline of the history. Tell what happened in the order that it happened. Maybe that would be less confusing? Give detail of why it happened and what exactly happened. Sort of like a biography, but it doesn't have to be that long, and it can just be an outline like what they do at school when you're starting to write an essay.
Hope that helped!
~ Moon