I'm not going to eat my boyfriend's grandmother's food. I just don't want to.
That's weird to turn it down. What's wrong with her food?
I'm convinced this way of preparing food had something to do with the deadly disease my boyfriend's dad died of.
Being picky on what you eat is natural. Though, if someone actually died from food poisoning, this should be taken very seriously. I would have a long chat with your boyfriend to make sure that his grandma is cooking everything properly and also about the true nature of his dad's passing.
I don't think the way someone dies is a topic for casual conversation.
Maybe look through the kitchen or garage to make sure she doesn't have anything that could poison someone. You need to make sure before you eat it.