So first thing I think when reading this is " Where are you catching the train? is it too work? is just going out?.."
next was wow these people are horrible! Poor Betty!
Then I was like reading the sort of person Betty is. No wonder they are avoiding her!

But then I went back to thinking these other girls are not very nice at all, Im also still stuck on where you are catching the train too.
Then the scapegoat thing.. No no no, don't let them do it. Betty sounds like she could get pretty nasty & if you work with her then it will come back & bite you.

Talk to your friends, tell them the way they are behaving is extremely pathetic & while you enjoy their company its really getting out of hand. Maybe talk to them as a group & then suggest someone other then Penny to talk to Betty.
Why Betty hasn't cottoned on is beyond me, the other girls have ditched her time & time again, one would think she would realise by now! Someone needs to tell her.