Is his will legal if he was doing chemotherapy and under a lot of drugs at the time?

A person can make a new will anytime provided he or she had legal capacity to understand the extent of his assets and the natural beneficiaries to his estate. The chemotherapy treatment, while debilitating, is not likely to have rendered your ex-husband incapacitated and unable to create a new will. Furthermore, upon the entry of a judgment of dissolution of marriage, any legacy or appointment granted you as the spouse would be revoked as if you had predeceased your ex-husband. This result is required by statute as it pertains to any will signed prior to the entry of the judgment of dissolution of marriage.

Any he left to you in a will written before a divorce is voided by the divorce. The fact that he wrote a will while in chemo does not in and of itself effect the validity of the will. If the chemo effected his ability to comprehend what he was doing, then it might invalidate the will, but that be of no use to you as a former spouse. You are entitled no nothing, unless the divorce judgment requires he do something with regard to his will, which is an unusual provision for a divorce order.