This sounds like a tough spot to be in. You say that she no longer goes to your school so you know that it is at least safe to talk to him and she would not be there to see or hear it. I think you have to decide, is he the kind of guy who would take this information, think about it and be flattered? Or is he the kind of guy that loves drama, would run to her and tell her everything (or all his friends for that matter) There are drawbacks to both telling him and not telling him. If I were you I would weigh out the pros and cons of both options and then go with whichever has more pros. I would hate for him to laugh or tell his friends or her and cause you drama. Remember, boys are not as mature as girls (scientific fact...and I know people are going to be mad that I wrote that, but its true)
Good luck, I hope it all works out.