What a terrible situation.
I would call my friend and arrange to take her kids to your house. Maybe for a sleepover and make sure her kids have a great time. I am sure they are feeling the loss too.
Also, I would spend some time with your friend. Call her, bring over dinner, take her out to dinner, do something that she enjoys to help get her out of the house.
I think it would be ok for the girls to play again. While they play, you two can talk.
My friend lost her husband about a year ago. Her biggest fear is that people will forget him. And she wants to talk about him whenever she can. That's just her; I would see what your friend prefers.
If she wants to talk and cry - be there for her.
I am sure she is needing a good friend right now.
P.S. And don’t tell her you know how she’s feeling if you don’t. Or everything happens for a reason. I hate it when people say things like that. Just tell her you’re sorry.