Sorry OP, the sentiment is noble (if a bit look at me aren’t I so generous and kind because I have so much more than you).
However, the chances of success, as already proven by the one failure, are slim, so actually you aren’t helping your friend. I have watched friends do this and I call it the doom cycle, multiple rounds of IVF, debt and still nothing to show for it other than lost years, broken dreams and, sometimes, relationships that never recover. Its why my DH and I never entered the doom spiral, but, rather chose to focus on building a life we love and, to be honest, I am glad we did that. Of course I have a wonderful SS who has been in my life since he was young and I am hugely grateful we have a great relationship, but he is not mine.
You come across as very childish in your responses to other people too, you couldn’t see that promising money that wasn’t yours to solely give (not that I would even commit this amount of my own money to a friend without discussing it with my husband) was unacceptable and felt hurt when others pointed this out to you, you are ‘cross at your husband’ for refusing when he has every right to and then you assert that, rather than accepting his position you will ‘speak to him again’ when this has already caused a rift between you that’s see him remove himself from your company altogether.
Remember you can be a millionaire on paper and have very little liquid equity in the bank, do you know know and where all the money is? Maybe your husband has invested it in places it is not immediately easily accessible. £8k is still a lot of money and, as others have said, if this doesn’t work (and it has more chance of not than actually doing so) where do you draw the line?
Just so you are aware, I also come at this from a place where my mother wouldn’t give my brother the money for IVF, after a long and difficult decision making process she concluded it was simply something she couldn’t take responsibility for. My mother is also a millionaire, on paper, as is my grandfather, that money doesn’t go as far as you think anymore and being a ‘millionaire’ isn’t what it once was, both have worked hard (extremely hard and sacrificed a lot given they both remember having little to nothing growing up) for that too, (my grandfather is 101 and still does a little accounting work to keep his mind ticking over).