*disclaimer...i've never been in a program, but I think I understand the dynamics in general*
Hmmm....personally I would be up front with him and say that both you AND your sponsor think a relationship is NOT an option. He was 'poaching the newbie', which is both irresponsible and dangerous. I would not want to put someone in a position where they are more prone to relapse, but he may not 'see it that way'. It was a setup because 'his ride' was at their normal spot....where they have been man times before. I'd keep my distance, and keep an eye out for the next newbie who comes in....because you know he'll pull that setup again.
-pedagogue