Unless there’s some reason for having him killed by poison you can give him a severe allergy to peanuts and the wife fill the capsules with ground peanuts.
Tetradotoxin from the pufferfish may work. I guess if the right part is cut, it should be fine (or not depending on your point of view.) Also a, if the crime is in the gulf, Karina Brevis produces brevetoxins, and is quite toxic. I don’t know if you could just filter it from sea water and put it in a pill though.
You can not just buy it for home use as far as I know. One would need to find a source, but there are many if you know where to look. If the murderer were to start a small plating shop, no doubt she would be the first person questioned in a cyanide poisoning. She would have to obtain it serupticiously.
The same MO was used in an Agatha Christie novel
Beware spoilers, obviously