I have heard about the personality change that affects some people after heart surgery. Men, in particular, sometimes get very feisty -- probably because they are angry and recognize that they are not totally in control of their destiny.
It's true that some people must be carefully watched after such surgery to be sure they are not overcome by depression, etc., which can manifest itself as extreme anger and an "I don't care anymore" attitude.
Get your loved one to a doctor, who may well recommend a psychologist and some meds to help smooth things out, at least for awhile. ]
Do this now -- (I know of a man in his late forties with a wife and two young daughters who had bypass surgery, did fine, then less than three weeks later went berserk and tried to kill the kids and himself. He did cut one of the girls, but not badly; however, he succeeded in killing himself right there in the living room.)
This is an extreme case, but weird things happen to people -- best to pay close attention!!! Good for you for showing such concern.