Hello lovesummer: I'm sorry you & your daughter are faced with this difficult situation. The Skeezyks doesn't have any authoritative information to share with you here. However I saw no one had replied to your post. So I thought I would.

My personal perspective with regard to this is that the best approach is to simply remain neutral as much as possible. My thinking is that if your daughter tries to challenge her bf's thinking, this is only likely to make him mad. And he's unlikely to accept what she says anyway. But by the same token, if your daughter's bf is engaging in grandiose unrealistic thinking, one certainly would not want to contribute to it by validating it. I would simply recommend staying out of it to the greatest extent possible. Sure, your daughter can try to change the subject if she can. But finessing that make be a challenge. And it may not always work. I wouldn't recommend trying to force it.

And then with regard to the potential danger factor... I'm not aware of any established relationship between manic bi-polar episodes & violence. But, as you wrote, you don't even know for certain if he's been diagnosed as being bi-polar. So you don't really know for certain what's going on. And, regardless of your daughter's bf's status (in terms of whether or not he's been diagnosed), violence can-&-does occur within the general population, including bf / gf violence. So it's obviously something that has to be considered as a possibility regardless of your daughter's bf's mental health status.

So this is my thinking with regard to your post. Hopefully other members, here on PC, may yet come along & offer their (possibly more experienced) perspectives. I wish you, & your daughter, well...