Refuse to enter any and all meetings without having your rep or another trusted member of staff present as a witness. Make exhaustive notes of what was said by whom and what agreements, if any, were agreed upon. Put this all in writing, 1 copy for yourself, 1 copy for the member being represented, 1 copy for the attending manager and 1 copy to be filed in the union office.
Do not believe their lies.
Always ask for any minutes of the meetings you do attend in writing and read them carefully checking for any discrepancies when compared to your own exhaustive notes. Do everything by the book and maintain your calm as management loses theirs. Record all this and how it all made you and the person you are representing feel. They do not get to tell you how you feel.