Sorry to hear, this is very sad. Couple of things that come to mind:
It's so kind and lovely of you to want to support your friend and I know you are coming from a really good place in asking the question, but do bear in mind that there is no single right or wrong thing to do, no linear path through grief or 'best way' to grieve or support someone grieving, everyone is different, every situation and loss is different and there's nothing you and say or do that will 'fix' this for her. I know that sounds stupidly obvious but it's quite a natural human reaction to feel as though if only you try hard enough and find the perfect words of comfort or the exact right practical thing to do everything will be OK for your friend (this comes from a place of fear and desire for control in our own subconscious) - but if you aren't careful it can come across as minimising or rushing the person through their grief or dictating what they can and should be doing or feeling. In many senses all you can do is do your best, knowing your best probably won't be enough. Be there for her, in whatever way you can, and follow her lead, in the acceptance that she may not be grateful for your efforts, she may not feel at all better or happier for a long time. I'm not saying don't do or say anything at all, absolutely not, just that you should have very low expectations of her response and be very tolerant /accepting that what you do may not seem like it's making much difference to her.
Along similar lines, I would say the number one thing is that whatever you do, whether that's regular phone calls, letters or cards of sympathies, practical help, making extra time for her, sharing happy memories of her husband, letting her know you are thinking of her and love her - keep doing it! This is the thing that comes up so often on this board, in the immediate aftermath of a death, particularly a sudden, unexpected or tragic death, the bereaved tend to be almost overwhelmed by an outpouring of love and support, people send flowers and gifts and cards, they make food and offer lifts and babysit children, everyone comes to the funeral and shares lovely memories, all wonderful, but then almost overnight it all stops and everyone goes back to their normal lives, often leaving the bereaved person feeling desolate and trying to come to grips with their 'new normal' without the lost person, whatever that looks like for them. Often people completely stop talking about or acknowledging the loss, frequently this is well intentioned or just from feeling worried about upsetting or offending, but to the bereaved it can feel as though everyone's forgotten or expects you to be 'over it by now', even if it isn't explicitly said. This time is where a really good friend can come into their own, just letting them know you're still there weeks, months, years down the line, being available to talk/cry at even at inconvenient times, offering practical help, acknowledging key dates like birthdays, anniversaries etc, letting her know her husband and her loss are remembered (if your friend would like that of course, don't do any of the above if she doesn't like it).
Basically I would trust that you know your friend best, you know for instance if she prefers phone call or texts, euphemisms and sentimentality or black humour or cold hard practicality, if she is a flowers person or would she like a more practical gift of food or similar, or if you aren't sure, ask her. Keep showing up, keep in contact, keep remembering it's not about you (although do in an appropriate way and time share your own memories of her husband and your sadness he's gone, when I have been bereaved I found lots of friends/extended family didn't want to 'burden' me by talking about their own grief which I do totally understand, no-one wants to be a grief vampire and make it all about them but in the end I found it very comforting to know the impact the person had on the wider world and how much they were loved and remembered and missed).
Don't judge her or imply she's 'doing it wrong' if she doesn't react how you would expect her to, or based on how other people behave in the same situation, I'm a bit of an upside down griever I've found, I tend to cope very well (on the surface at least) in the immediate aftermath and the days/weeks following a death, particularly if there are practical things to focus on or other people needing my support. I then tend to go through a snappy/irritable phase as things settle down, when nothing is right and no-one can say or do the right thing and I push away any offers of help/support, then the massive, existential crisis melancholy/sadness hits months and months down the line, annoyingly right about the time everyone (work, friends, wider family) are starting to think I really ought to be moving on and going back to normal, just when I really could use some love and support. I know I'm a nightmare to deal with and not saying your friend will be the same as me, but cut her some slack if she turns quite unreasonable or difficult to be around for quite some time and in quite unpredictable ways afterwards!
Hope that helps 