Maybe you should ask your daughter what she knows about his religion. Most of the time, young people don't go into a lot of detail. Just say you were interested in finding out about his religion since you weren't familiar with it.
If you do it carefully and without sounding biased, you can show how they don't accept blood transfusions etc. Ask what she thinks about that.
Teenage years combined with the "first boyfriend" ever, might make it more difficult for her to see the danger in this religion.
But if he is into the JW religion and wants the approval of his family and congregation in order to marry her, he will work at getting her to study and commit to the religion.
There is a great place to find out more about this religion if you haven't already found it - https://www.jwfacts.com/
Hope it turns out alright for you and your daughter.