I’m very sorry to hear about Cupcake. He sounds like he was a cute little bunny and very dear to you.
You absolutely cannot take any blame for any of this, and I hope you will stop thinking that way. If anything, you provided some education to this family to start them on their way to making a better home for Cupcake. If they didn’t bunny proof their home it was due to lack of effort on their part, even if you believe they did it to the best of their ability. The could have asked more questions, read up on rabbit care, or researched on line. They could have talked to a vet. I don’t mean to disparage your friends and neighbors, but clearly they could have done better, and to blame yourself is just wrong.
Cupcake was in the care of their grandmother. Again, though well meaning, she was focused on attention rather than being informed of what are important health indicators. She was responsible for noticing that he hadn’t eaten, and yours was a natural assumption. I’m sure she feels really terrible that he passed while in her care. Don’t make it harder for her or yourself by imagining that you should have spotted what she didn’t. It takes education and observation to know the subtle signals a bunny gives off when ill, and I would never expect a visiting family member to know that without my explicit instructions. Even then, an experienced bunny owner can miss the signs because bunnies hide them. It is very sad, but neither of you are to blame.
Cupcakes has crossed the Rainbow Bridge, knows how much you cared, and is happy to have a place in your heart. He will be missed.