First of all, you praise him about his behaviour when he consider about blood issue. Tell him you admire him eventhough he is a nurse, ever believed (and maybe still believe in) blood transfusion will help in critical occasions (Intensive care), but he also consider other side of the matter by asking JWs' viewpoint about blood transfusion.
Then, now he may consider about religion (JWs), you wish him also has a moderately behav', mean he should know opinion from another side too. You give him the book "Crisis of Conscience" by Raymond Franz (amazon.com has it).
You want it or not, if he turns into a JW, your lukewarm situation will be reported (for he already know you ever been involved w/ JWs and now fade away). If you tell him about Watchtower is a cult and he agree, you have your friend back, if he refuse your advice, it's up to him. Even you keep silent, not telling him JWs is a cult, he still report you anyhow. That's my (humble?)opinion
jwsons