The RSPCA always take photos before they remove any animal, so they can be used in court. I would suggest taking photos, then call the RSPCA. Don't tell your mate you have called them or he could reactly badly and kill the snake to hide the evidence.
The should come out and take their own photos, and then take control of the animal. Depending on the severity of the case, he will either be fined or go to court, they may even suspend his licence, at least that will stop hom owning reptiles (at least legally).
Hope some good can come of it, and you report him ASAP!!