Hey OP, I would be in a similar position to your daughter - my dad had Huntington's and died a few years back now. He was diagnosed in his 60s and actually went on to die of lung cancer, not the disease we'd all been obsessing about for years. I know it's hard but it just shows the futility of worrying about it.
I've been to genetic counselling - I went when I was pregnant with twins. I was eaten up by the fear of passing the gene on to them. I realised I didn't want to terminate just on the possibility that I MIGHT pass the gene on, particularly when so much progress has been made. We don't have a cure for it yet but last year there was a massive jump in gene research which scientists believe may hold the key. For the first time since I got the news, I felt positive that if my children ARE affected by the gene, there may be hope for them.
My dad led a full, active and rich life and only got diagnosed incidentally when he had a stroke (he was a heavy smoker) in his mid-60s. Although in the following years the disease developed, his life wasn't cut short as others are by HD.
I was told that due to the pattern of inheritance, I couldn't pass on the juvenile form to my unborn children. To me, that was the most important factor. If they do get it, the chances are we're talking in four or five decades time - I feel genuinely optimistic that we'll know enough to manage/treat the disease by then.
I've not been tested. I'm normally the person that likes to face things head on, but really I felt there was no point. It's very easy to become obsessed with this disease and to second guess every thing that happens. I may well have Huntington's, hopefully not, but maybe I do. At the moment there's no obvious signs and that's what I choose to focus on. I'm 44. I know how hard it is, believe me I really do. I've been on a hugely emotional journey to get to this point of acceptance. I'd really try and urge you to think about the full range of possibilities rather than just the worst case - your daughter may be positive, but there may be a cure (I'll find you a link to the research and post it separately). Your daughter may be totally unaffected and test negative for the gene. Your daughter may not show symptoms until very late in life. Don't waste your time panicking about something that might never happen. And I hope that doesn't sound patronising or flippant - it's taken me a long time to get to this point.
Also, and I hate to go back over old ground - but why was he tested? You aren't aware of either of his parents showing any signs and according to the NHS letter he is asymptomatic. When he was diagnosed with BPD no-one raised Huntington's then. Why on earth has he suddenly been tested? It's not a simple test and they don't just let you have it because you fancy it. It just sounds.....odd. If his psychiatric symptoms were due to the Huntington's, it's unlikely that he'd be fine now. They're not isolated psychotic episodes - Huntington's is progressive. I used to work with the research council in Wales for Huntington's and also used to be part of the local HDA so I've got a fair bit of experience.
As for tomorrow, there's going to be no sudden revelations. It's just a chance for you to ask questions and get information so think about what you need to know in advance. Be as honest as you want, they understand how shocking this kind of diagnosis is and the impact it has on the wider family. They won't judge you.
Do let us know how you got on? If I can help with anything at all, please feel free to inbox me. Sending hugs.