Re: Going out with a girl who said in advance "I've got a BF"
She does NOT sound like the kind of person who would go back on or cheat on her boyfriend, if that is what you are intending. She doesn't party, and she doesn't drink, which are two flags that say she's locked down tighter than a prison. Heck, I may even go a step further in saying that BF Destroyer will not work on her.
If all you're looking for is friendship from her, then you are more than welcome to do whatever the two of you want. If not, then it would be wise to move on to other women and the bigger scheme of things.
"There is no better than adversity. Every defeat, every heartbreak, every loss, contains its own seed, its own lesson on how to improve your performance the next time." - Malcolm X