So to your initial question and point that you don't want to get the current owner into trouble, ask them if they have a CPH number and flock number. If they do it is simply a case of ordering some ear tags and getting them on the sheep and then a movements licence.
If they have a CPH number but no flock number they need to speak with the local Animal Health office to get one, and if they have neither number ditto. If they don't want to do this then to be legal you would have to stay clear of these sheep.