@Chillies I think tell her when you're ready and feel more comfortable, maybe after 12wks, don't feel pressured to do it earlier for her sake. I would word it something like
"I know this may be hard for you to hear, but I'm pregnant again. We hope the outcome will be better this time but we obviously can't predict how this will progress so if you could please keep it to yourself, but I wanted to share this with you as my close friend".
With my last pregnancy (MMC) I told my friend a month later after we lost bub at 8 weeks, and she was more interested in when I found out I was first pregnant, and that I didn't tell her!!! Unbelievable!
Here I was mourning the loss, and she had v little empathy and made it about her!
Look after yourself and your bub Chillies, good luck