Option A
If it was me I would flog both tickets and give the other girl her money back but your so called friend gets nothing as she still owes you £400....But that's if you're feeling brave enough and you're prepared for a shit storm.
You could then send a message saying you will deal with their request when you are ready and then block them until you've sold their tickets, then give the money to someone who can pass it on to them or, if you have either of their bank details, send it that way and don't deal with them in any way again... block them on everything.
Option B
Do you have other friends who would be willing to buy the tickets off you and then you can go with them and post pictures all over social media of your good time. 
You can give them BOTH their money back and make it clear that as their behaviour has stopped you from going they needn't think they can go. You can wipe your hands of them and walk away with your head high.
Option C
Sell all three tickets, give them their money and spend your money on a Spa day.
Be firm, don't be bullied and don't let on until you've done it. Whatever you decide you can inform them on the day and tell them: I'm kind but I'm not a mug and your actions have consequences. I've sent your money back, enjoy.
I never give someone another chance any more because often it's like giving them a bullet for their gun when they missed you the 1st time.
P.s
If person B is the other girl.... she is not your friend either. Get new friends.