High School reunion - what to say to my best friend's stalker?

My 5 year High School reunion is coming up very soon, and my best friend (a female) is unable to attend. There was a guy in our class who started stalking her soon after graduation, and continues to try and contact her even 5 years later. (I won't go into a lot of detail, but she, being incredibly shy and naive, rejected his offer of a romantic relationship, and only wanted to remain friends. He continued to pressure her, and she cut off communication when it became too uncomfortable. He just won't take "no" for an answer.) The inventive ways that he has utilized to contact her just amaze (and scare) me. Needless to say, I try to keep an eye on his whereabouts, but he doesn't have any money to travel, and she lives 20 hours away now, so we think she's safe.

My worst fear is that he'll actually COME to the reunion and start questioning me about her whereabouts. If he does so, what should I tell him? I've thought of telling him that she won't be in town this summer because she's doing an internship overseas. I need to say something that is final, but not offensive. The last thing I want to do is offend someone that could be mentally unstable (emphasis on "could"; I'm just going off of his stalker habits over the past 5 years).

Anyone have any ideas? Serious ideas only, please. I am not going to tell him to "get lost" as that is just rude, and will probably only make the situation worse.