Urgh - counterfeit money. I ran a small cafe for a few years, and we were warned about counterfeit £20 notes going round. I only got fooled once, by a counterfeit £50 when I was very busy and stressed - and very broke. I should have known better, as the man who handed it over looked very dodgy - in fact I did the transaction extra fast as I felt uncomfortable with him in the shop.
Most gas stations in the UK use scanners for all notes, so counterfeits are detected right away. Glad you and your friend learned from the experience! Thanks for sharing.