She may not have the capacity to tell you what she needs right now, and asking might add undue pressure. She'll probably be in shock. So think for her instead, she's your best friend, tell her what you're going to do for her so she doesn't have to think for you.
Cook dinner, school run, childcare, make a cup of tea, give her a hug, washing, cleaning, kids bedtime, anything practical.
I went through this, my BFF DH. They had 4yo at the time and were living with my BFFs family. Awful experience to go through. They'd been together about 18yrs. The whole family will be hurting. Your friend's parents, friend's siblings too probably.
Tell her you are here for her. Remind her she is loved, they all are. And remember the special dates like his birthday, their anniversary, a couple of weeks before the anniversary of this.
I'm so sorry your friend and you are going through this too.