I think there are a few possibilities for anyone being at fault, but if they weren't mentioned in the lawsuit, they are probably non-existent:
1. If there is some kind of training for security guards which specifically states that you must diffuse these situations in "x" way to prevent possible heart attacks or stress escalations, then they could be in trouble. Likewise, if there is some kind of memo or information that was sent to the NFL, the Patriots, or the security company, which informs them of the "risky" practice of escalating arguments with fans, then they could be in trouble. However, I doubt there is any correlation between getting into an argument and having a heart attack that is specific to these particular event versus any other type of life situation.
2. If the deceased had said something to the security guard along the lines of "I have a condition, please leave me alone" or "I had a heart attack, please desist" then someone could be in trouble. The fact that he was obese would actually work in the plaintiff's favor, not the defense, but I don't think it's enough- the fact that the man was at a sporting event and apparently was animated himself would negate his supposedly identifiable high-risk obesity. It's not like the man was in a wheelchair and the security guard pushed him over.
Medical outcome, or injury, has to be reasonable foreseeable. I saw the definition that you posted on here, and maybe proximate causation is not the right definition in Massachusetts; many of these legal terms mean something else in other jurisdictions. However, the concept that something needs to be reasonably foreseeable exists in all jurisdictions of common law, and I'm sure it is called something else in MA. Otherwise, I could be negligent by leaving a gun in my front yard, and an unlikely hail storm comes, hail drops on the gun, discharging it and killing someone. I was negligent and the person died, but I would not be liable because that would not be foreseen by a reasonable person and was just really bad luck. I would be liable if there was a school across the street from me and a child picked up the gun and someone. Obviously a reasonable person would know not to leave a weapon out in the open when there are children that frequently walk by.