""I went to school in Ireland and I don't understand a single thing the OP""
WTF is going wrong in Ireland?
I was educated in the UK and have understood everything the OP has written and more importantly I understand (partly) her dilemma.
However that might be because I'm not a piss taking twat, who likes to kick someone when they are down.
OP, this friendship is causing you way too much stress to be positive, rethink it.
This is a fantastic opportunity in the current job climate, take it.
Remove everyone who isn't a true friend and wouldn't want you to go for this, from your life.
You don't need this shit, you'll find many things will fall into place once you do what you need and benefits you.
Plan to go to the interview, tell your friend, if she isn't happy for you, she's doing you a favour, by cutting you off.