Conman Rakesh Bhayani has been jailed for life with a minimum term of 27 years for murdering County Durham woman Carole Waugh.
Bhayani stabbed Miss Waugh to death and went about spending hundreds of thousands of pounds of her money.
More top news
The union claims the company is trying to push through a package which would threaten jobs, working conditions and safety.
The mum-of-two wrongly claimed income support, housing benefit and child tax credits between 2010 and 2014.
A 17-year-old boy was pronounced dead by paramedics at the scene while two other men were taken to hospital.