I think that is a right he has cbg.
Most union contracts, in my experience anyway, have an agreement with management, to reprisent ALL employees, regardless if they are paying members or not. I would approach the head of the union at your facility, and see his/her thoughts on the situation.
U may be better off to become a paying member, if they decide to take your case. Question #1 would be, did you write the statement, B. Do you have any proof that you could NOT have seen the employee in question, acting different, or strange in any manner.
If you weren't in his work area, to witness him intoxicated, the employer has NO right to threaten your job, if you didn't witness any event.
If you were in the work area, or spoke to the employee, it is the employer's right, that you submit a statement of what you witnessed. If you didn't see anything, that's what I would have written on the report.
An employer does retain rights to "force" you to witness I guess, but not lie.