It is a lovely gesture, and they obviously think very highly of you. As the lady wants you to have the ring as a gift, you should be entitled to keep it. However, as others have mentioned, if it is of value, I would want make sure that her son & daughter are aware of the fact that their mother wanted you to have the ring.
Sometimes, unpleasant things happen when people pass away, and it would be dreadful for you if, at some point in the future, her children want to know what's happened to the ring. I'd be inclined to get it valued, and if it is worth quite a bit, I'd tell your friend how much you appreciate the gift, but would you'd feel more comfortable about accepting it is she tells her children that she wishes for you to have it. I'm sure, knowing how well thought of you are by their parents, they would be more than happy for you to have it. But, assuming they are aware of the ring and, if it is valuable, it would be awful if they wondered what had happened to it after their parents passed away, and that could put you in a potentially difficult situation. Get it valued, and then you'll know if you need to voice your concerns to this lovely couple.