I would say that it depends what type of heart problems he has. Grapefruit juice is not recomended for people on certain medications, so I would use lemon juice for his cleanse.
Epsom Salts are Magnesium (Mg) not Sodium, so the shouldn't negatively affect his blood pressure. Magnesium is known to relax smooth muscles, such as those that contract bile ducts, arteries and veins. So, the ES may lower blood pressure. Calcium/Magnesium balance is very important. Normally, people with heart trouble have more calcium than magnesium, so again, it is probably ok to take a massive dose of Mg. The body flushes the excess -- that is how ES act as a bowel purgative.
The question with diabetes is one of blood Sugar monitoring. Not eating from 2pm until 10am the next day could be a problem. I'd keep close track of the blood Sugar reading. The large dose of OO (Olive-Oil) and lemon juice provides some nourishment, but it is unpredictable how diabetics react. The critical thing is not having blood Sugar drop below about 75. If he starts sweating and gets confused, he needs some orange juice or something to raise his blood sugar.
In general, if he can tolerate a series of cleanses, it is likely to help both his cholesterol and sugar metabolism. The liver is key in both regulation of lipids (fats) in the bloodstream as well as glucose. Stimulating it with a cleanse may help him lower his medication levels over time.