Whether you agree with JW theology or not, if you are in their meeting house, church, or whatever they call it, you should be respectful.
You are a guest there, and should conduct yourself appropriately. With that being said, if they should start attacking another faith, whether it be Catholic or not, then I would repectfully excuse myself and leave. You can make a simple, respectful statement of ,“I’m sorry, those are not my beliefs”, and excuse yourself.
As Catholics, we expect visitors to a Mass to be respectful of where they are at, and what is going on. We don’t require they believe it, we simply ask that they respect it while they are there.
We should offer that same respect to individuals of another faith whether we agree with it or not, especially in their house of worship.
If you are so eager to attack them in their house of worship, my recommendation would be not to go. Simply explain to the individual(s) that invited you that you cannot attend in good concience, based on what you know about their theology. Of course, do this before you enter their hall.
Kindness and respect will get you much further than anger and attacks.