This sounds like it’s going to go South quickly to me. It’s exactly the type of deal we would run from the hills from when I worked in Asset Finance and recoveries.
So your foreign friend gave £15k to someone he didn’t really know, who didn’t give any sort of proper paperwork outside of a non-branded word created proforma, and the guy is shady, uses multiple addresses and trades using a mobile number.
I hope I’m wrong, but if it all goes wrong, ignore small claims court, it’s easy but they’re pretty toothless against the truly belligerent, get a solicitor and go for High Court, have you seen “can’t pay…” they don’t **** about.