Sad to hear, but as to the cause, not likely the welding machine. I have worked with them since 15 years old and although you can get some mild jolts when wet, sweaty etc and touching the ground and the electrode, nothing in the way of enough to cause electrocution.
My bet would be 110V lighting used in the enclosed space vs the OSHA required 12 volt DC lights. Either way, if the widow is smart, she already has a lawyer working on it. OSHA will find the cause and should already be onsite investigating as they always do when a death occurs. Lord help the company if it is discovered that they have participated in a cover-up to the actual cause of death.