He has the choice to stay in the marriage or leave the marriage. (In fact, he always has those choices regardless of whether or not she is diabetic - there are no regulations about why he can or cannot divorce).
The reality is, they are already married. It isn't possible to go back in time and retrieve the time, money, effort, emotion, social investment, and so on, that was put into the marriage. Even in a bad marriage, divorce usually brings some amount of suffering. The only question is whether divorce is better than remaining in the marriage.
It seems to me there are two issues here: (a) the health condition itself, and (b) loss of trust (in her/her family, depending on who is responsible for the omission).
Regarding health, there are no guarantees in life regarding health, and most people, if they live a long life, at some point develop some sort of health challenge.
Regarding trust, it is not possible for an outsider to say whether the man can forgive this and move past it, or not. While lying by omission is ethically wrong, and it is a shock to find out one's spouse is hiding something big, I would say that both men and women frequently lie about or hide things before marriage. I have known people to lie about their citizenship, their age, their real legal name, their sexual orientation, previous marriages, other wives, their biological children, and even when they were Sunni or Shi'i. The list goes on and on. There is a saying in English, "Keep your eyes wide open before marriage and half shut afterwards". Such is the nature of this world and the human condition, whether out of fear or seeking an advantage, people do these things.
Islamically speaking, it is wrong to lie or be deceptive. However, it is also better to avoid divorce (where possible) and to forgive.
It might be worth considering the effect of divorce on the man and woman, should that be the decision. It will likely have at least some cost for them - financially, socially, emotionally, in terms of time lost, etc. There is no guarantee that the man will find another wife that he will be happy with, and a new wife could easily come with other challenges (like difficult relatives or a temper). Also, it is thoughtful to be considerate of the situation of the woman. In some social circumstances, divorce can be like a lifelong punishment for a woman. I am not sure whether or not this is the case here. Of course, it is not his responsibility to fix all the ills of the world, but it is good to be thoughtful of it, especially if he cares about her.
Anyway, since the two are already married, why not give the marriage a try and see how it goes? For instance, see whether they like each other, are compatible with each other, would he have wanted to marry her anyway, can they see themselves spending their lives with each other? If they are happy, there is no reason to separate.
Giving it time will also allow him to see whether her being diabetic is something he can handle with respect to practical issues such as medical care/medicine, dietary needs, or whatever else may come along.
In any case, it is good to come to some closure on the issue. If they do stay married long term, it is not healthy if the issue keeps popping up. It is not unusual for spouses to continually bring back old issues during disagreements, even when those issues have nothing to do with what is going on. It is healthier to address it and then lay it to rest.
Also it never hurts to pray for guidance.
