1) It is upon the person themselves to determine whether fasting will have a negative effect on their health. Hence if there is a good possibility that by fasting, the person will fall ill then fasting is not wajib however if that probability is weak, then they have to fast.
It is upon the person themselves to assess their situation and act according to the rule above
2) If the person fasting gives a strong probability that fasting will cause them to get sick then they should not fast and if they fast and get sick because of fasting, then the fast is problematic and qadha has to be done (if they have the strength to do qadha). If not done, then in addition to qadha fidya (750 grams of food) should be given to a poor person for every missed fast