Mr Williams left his wife and their 12-year-old daughter in the UK on 7 May to travel Ukraine.
The family said they lost contact with him on 2 July after he set off on a mission, and were later told on 14 July he was listed as missing action, and two weeks later a member of his unit confirmed he had died.
Ms Boyce-Williams told BBC Radio Merseyside: “I was sitting at home with my oldest son when I got the text through to say they had important information and could they call me so I knew instantly.”
She said she and others in the family had tried to convince her husband not to go to fight but he was determined.
“We were all absolutely devastated when he told us [he was going to fight],” she said.
“We spent time trying to convince him not to go, telling him how it would affect us and how it would impact our daughter, and everything else back home.
“But he was such a strong-willed man that once he got something in his mind there was no way you were changing it.”
“He wanted to help civilians, especially the children,” she said.