I am not a union rep, but am clued up on things....

They cannot make your husband change shift without 4 weeks prior notice and only then a good reason is needed. If they are accusing him of something they should really suspend him until a full investigation is carried out. It is a shame he is not a member of a union as they would do all the leg work regarding this. Personally I would politely refuse their request and ask them to show proof of wrong doing. If they can't then he should stand firm and refuse the request.