I don't know how well you know your neighbour but if she is a friend then believe in her. The experience of abuse makes people feel worthless and it's this more than anything that keeps them at risk. You can help her most by being there for her whatever she decides and giving her support, encouragement and information. Her husband may be willing to get help too.
If you are in the US, here is a link for people she can contact who will assist her:
Get Help
If she decides to leave, you should know that often women leave several times before they can make the final break. One of the problems is that friends try so hard to protect them that they feel betrayed when this happens and the abused person feels they need to take sides. Often, when they are most at risk, they are isolated with the abuser and don't feel they can contact people.