My ex wife has taken in a lodger who is a Jehovas Witness. My ex is not a practicing Catholic and has no strong faith views. My daughters aged 10 and 15 often stay there and will interact with the lodger (24 yr old girl). Is there anything I need to know or be expecially concerned about?
Yes, you should know that they have been extremely well trained to pick apart the Bible (even changed some words in the Bible to support their claims) and Catholic teachings in particular, so as to debunk many teachings. I wouldn’t try to have a Bible verse war with them, because they’re well trained as it is a focus of their teachings, while Catholics are not hang on every syllable of the Bible anyway and so won’t know the verses or understand them enough to spar with them. But any Catholic with a strong and solid foundation in Catholicism would not be affected. But they really do get a lot of Catholics to convert due to typcial Catholics’ lack of knowledge of the Church. JW’s don’t just try to debunk Catholic teachings, and try to use passages out of the Bible (usually their version of the Bible) as backup, but also try to debunk teachings that the Church doesn’t teach

So a Catholic who doesn’t know their faith doesn’t really stand a chance.
If you’re going to counter any arguments, be prepared for a very prepared person, and try to delay the match until you can research it. It’s fair to be allow another person to research their material before discussing a subject, so they should be agreeable to it and they usually are. After all, they’re very well trained and look forward to these discussions. Insist on using a Catholic Bible, since that’s where the Bible came from anyway. Some JW’s actually think theirs was the first Bible that came to existence. They also try to divert to another subject when their arguments get weak, so try not to let them divert you, hammer out one at a time.
If I were you, and I"m not you, if at all possible, I’d absolutely try to limit exposure of your kids to this JW because yes, they will be in danger of converting if their mom doesn’t defend them and their faith. If that means them not spending time there often, then so be it. They’re too young and impressionable to tangle with JWs. The JW’s I know can be outcast if they don’t try to convert their peers, which is not the same as Catholic ex-communication. The emotional blackmail that comes with this shunning is very effective.
I don’t really care if some of ya’ll think it’s extreme, but I’d curb those visits as long as the JW is there and if my ex would give me any trouble, I’d call a lawyer to help get a legal stop to any conversion opportunities. I wouldn’t allow any religious discussion in front of my kids if I had any say in the matter, not without you present to help them sort it out. But I don’t know what your legal position is.