If you have a close friendly relationship with her, then a friendly chat with her about a few things about the JW religion, might work.
"I wanted to know more about Boyfriends religion and found out about the Holidays. I also found out they restricted your type of clothing (that might get her the most). Sounds funny doesn't it? If you wanted to watch the latest movie about ---- you wouldn't be able to go because of it's rating. Even dating Boyfriend can cause a lot of trouble in his place of worship because the men in charge - Elders - would say he is disobedient being with a "worldly" girl. They have all the power in their church and they make sure each member does what their religious leaders tell them to. They are like Scientology in many ways."
Only you know really what would be appropriate and right to discuss with your daughter, but you'll regret not saying anything.
This religion can have a strange hold on people. Even if they have been out for years, the fear this religion instills in you, can drag you back in. Especially if something frightening happens in the world. Then the thought of Armageddon and God bringing judgment on them, will send them panicking back to the safety of the religion.
I certainly hope that Boyfriend leaves this religion, but although the parents seem nice, they are still in. Their child's future will be one of destruction if they allow him to leave. Sad but true.
I had friends that I knew since I was 13. They stopped going to the meetings for years so I would bring them the latest magazines. They only attended the memorial once a year. They even had "worldly" friends they went on cruises with and didn't mind helping out at Halloween for a cousins child. Yet when I found out that this religion was false and tried to tell them about it, they snapped right back into being Ultra Jehovah's Witnesses and condemned me.
That is what you have to be concerned about.