My ex-boyfriend is not following the Lord anymore?

You're not crazy for wanting to help him, but perhaps the best thing you can do is pray for him and let him know that you still want to be friends with him and keep the lines of communication open. After that, you have to let him live his life. If she's pulling him away, he may not have been very committed in the first place and was simply doing what he thought or knew you wanted him to do.