This isn’t just research. This is a reality that I’ve had to address. Research is by no means conclusive but it does show that heart surgery releases particles of plaque into the blood stream that can travel to the brain. This is partially where my comment about your friend not being the same person for a while came from. On top of that, who knows what the required medications a heart patient has to take do to the brain and one's personality.
Not tough love but unconditional love.
I hope this short account of my personal experience helps you to help someone who will undoubtedly cross your path with this open heart surgery issue at some point in your future.