Betty is just "Connie's best friend" for much of the story; we only find out her name from Arnold, who is trying to build his credibility with Connie by naming her friends.
All we know about Betty is that her father often drives her and Connie out to the shopping plaza at night:
The father of Connie's best girl friend drove the girls three miles to town [...] and when he came to pick them up again at eleven he never bothered to ask what they had done. (5)
But the fact that Arnold brings up her name suggests that she may have given him information about Connie...and may not be such a good friend after all.