That is so so tragic and heartbreaking. My sister-in-law's son had a similar experience when his friends' (at Brother Rice) mom and dad died in that explosion in---I think it was Wisconsin a couple years ago. They were there vacationing with the whole family for a big family reunion. The grand parents were there and were injured, but recovered and now take full care of the three children (2 teenage boys and a young just-teen girl).
My sister in law just poured on the sympathy, condolences and support in any and every way possible. She did not wait for anyone to show a sign they wanted to talk, she just asked them how they were, asked the kids over for days/nights at a time, went and cleaned their house, etc. The whole community just rallied around them in the same way. If I were in your heartbreaking shoes, I would do the same kind of thing and encourage my children to do the same. If they (her/the kids)don't want to talk about it, you will sense it and then can back off. And even if they don't want to talk about "it", they may want to talk about other stuff. And if they don't want to talk at all, they may still not want to be alone. So I would say create opportunities for your son and your friend/her kids to be together. If you sense she/they need alone time, you back off a little. It's a time where you have to listen...even if they are not talking a whole lot...trying to pick up on their cues. Prepared meals (for freezer) and cleaning (cleaning ladies) would be helpful. And cards with handwritten heartfelt thoughts are ALWAYS far more comfort than most people would think.
Also...my same sister-in-law's own sister-in-law just participated in a fundraiser in Canton for a wife/family that lost their dad. Is this the same family? My SIL said they raised $12,000.00. If this is the same family, could another fundraiser be arranged in November for the sake of Christmas? I'm just assuming it is the same family. If not, a fundraiser (obviously) is another suggestion.
You are right to remember that everyone grieves differently, but just keep supporting and taking their cues.
Our prayers are with them (truly).