Hi B.,
All advice below is fabulous. You are already doing the right thing by caring and wanting to help. Just be there for her, make it known that you are there to listen or be a shoulder to lean on whenever she wants to, without judgment and letting her go through her feelings as she needs to. She'll go through all sorts of feelings, including rage (how dare he let me down with the kids so young, stuff along those lines); just be there for her and let her go through. Grief is harder when there is no space to express it, and know that it can take any amount of time as well as any form.
In the short term, I'd be there to help her with chores, taking the kids, things like "hey, I made this stew, and I made way too much of it, do you want some?" Offer to take her children for the afternoon. And keep doing this for at least a year... Most people are there during the first month or two, then disappear, and it's often only 6 months into it (or even more) that the worst hits. When my sister died, many people were there to talk to my parents and invite them, but 6 months later, when they were really down and could really have used the company, only one set of friends was still there, inviting them out every once in a while, giving them a space.
As for her little boy, give him the space to express his feelings, and if he expresses anger, respect that too. It's a normal part of the process, and if you tell him that it's normal that he feels this way, he will go through the process much better. Kids tend to naturally have a much healthier grief process than we adults do, so if you let him go through it, validates his feelings (it's ok to feel this way, it's normal to feel this way, etc.) you'll give him the authorization to feel them as they come to him. But again, don't force. He may not want to share, and it's only when something boils over (like anger) that you'll have an opportunity, and it may be the only one.