I would imagine hes in the wrong.
Ive worked in jobs where you had to come in monthly for staff meetings and you were only paid the meeting time hours.If its a safety meeting then technically without the correct training they could say he wasn't adequately trained for the job.
The bottom line is in today's job market , if you've been happy for 3 years then to risk your job because of this would seem foolish.
If the company allowed everyone to miss the safety meeting then there d be no one that knew the safety rules/ training.
And if no one else was paid extra why should your husband be ?
Basically if i was him id go to the meeting state his case , calmly and quietly , and hope when hes disciplined , which he will need to be , that he only gets a verbal warning.
Training or staff meetings aren't normally classed as overtime in my experience.