I wouldn't worry about it too much, I have yet to meet a Diamond Way practitioner that eats babies or takes part in human sacrfices. Generally they are a pretty amiable and harmless bunch.
"When one is not in accord with the true view Meditation and conduct become delusion, One will not attain the real result One will be like a blind man who has no eyes." Naropa - Summary of the View from The Eight Doha Treasures
Thanks, folks! The person in question came back from the retreat unscathed and fairly unimpressed from what I've heard. He's been frequenting a Theravada centre for some years, I believe and Diamond Way is probably more unlike Theravada than any other Tibetan lineage?