Canon lawyer Cathy Caridi has actually discussed this very question on her blog, Canon Law Made Easy.
To briefly summarize her excellent article, canon law, following Catholic theology, does not recognize divorce as the end of a marriage, and so not recognize the validity of a marriage while either potential spouse is currently married. It follows that Ann and Chris are not, in the eyes of the Church, married at all. And of course it follows from that that there's no second marriage to become valid.
What Ann and Chris need to do to "regularize their situation" (that is, to ensure that they are living within the rules of the Church, but more importantly that they are living their lives rightly before God) is, firstly, to talk to their parish priest. It is likely that he will want to discuss the whole situation with them, and understand how they came to this point and what they understand of the Church's understanding of marriage. They will need to go to Confession and confess their sins, including the sin of adultery (which they committed by living together while Ann and Bob were still married, that is, while Bob was living) as well as fornication (which they committed by living together without marriage after Bob's death).
Then what? They certainly have an existing relationship, which they presumably believed was a real marriage. What they have to do now is to get that "putative marriage" (as it's technically called) validated. This can happen through the processes of radical sanation or convalidation (the likely next step in this case). How this is done is well explained in Andrew Leach's answer here as well as in other answers about validation of a marriage throughout this site.