There is a role for none related people to help raise children. this is the traditional role of the Godfather. Not to be confused with the jowly guy who makes you an offer you can't refuse.

A bit of a movie cliche but close service members sometimes form informal pacts to take care of each others children if something happens and one doesn't come back. this can be anything between a quasi-legal commitments and simply showing up and asking the widow if there is anything they can do.

From your description I don't see any such expectation from your friend.

At most I would think you might show up on her doorstep, express condolences, and explain that you were his friend and simply ask if there is anything you can do to help her. Done in a subtle and sensitive manner, being careful to avoiding offending her, you might help but you should be careful to keep a retreat path open and an eye toward backing off quickly if it goes bad.

Handle it wrong and your looking at a lot of verbal abuse, the possibility of ballistic room fittings coming at your head, and a restraining order. And a story to spill beer over.

Simpler and safer to keep your distance but little good ever came from playing it safe.