What an epic failure on PayPal's part. Is there some banking law that I'm unaware of that would cause them to take such drastic actions? It sounds like the OP is definitely above the age of 18 now too, so why would PayPal suddenly decide to yank the rug out from under him? It'll be interesting to see if there's a follow up.
As a partial update, after spending 30 minutes clicking every page of the dashboard, it was obvious that money was stuck, and my friend couldn't dispute it as it was sent as a gift, as it wasn't payment for work. Luckily, if you head back to your settings and flick back to the old web UI whilst it's still available, the refund button shows up in the view transaction details page, and it let me refund it. I lost the £20 or so in fees, but at least I can pay some unexpected bills more easily now.