What you did when your neighbor's husband died was good; it's important to help people and encourage them when they lose a loved one. The Bible tells us to "Rejoice with those who rejoice; mourn with those who mourn" (Romans 12:15).
But I suspect that almost everyone who's lost a loved one would tell you that the hardest times often come once the initial shock has worn off, and loneliness or depression starts to take hold. Unfortunately, this often happens just about the time people stop coming around and yet that's when a grieving person may need our encouragement the most.
This is why I urge you to reach out to your neighbor. Yes, I know it can feel awkward but what she really needs is your presence, not just your words. In fact, sometimes the best thing we can do for someone is just be a good listener. The Bible says we "should be quick to listen (and) slow to speak" (James 1:19). Don't feel also that you always have to do something big (like fixing a meal); she may appreciate a trip to the store or a cup of coffee just as much.
Most of all, pray for her, and ask God to help you point her to the hope we have in Christ. When we know Him, God truly becomes "the Father of compassion and the God of all comfort, who comforts us in all our troubles" (2 Corinthians 1:3-4).