Sixer good to hear an update.
His best option would have been to refuse, point out the consultation they are required to go through and have failed to do, and leave the ball in their court in terms of action to take, which then may well have been unreasonable of them.
But hindsight is a wonderful thing isn't it!
Difficult to know how the meeting will go. It may be that if the circumstances will allow, he should retract his resignation and go back to work, but make it clear he does not accept the change to his terms and conditions. As you will have seen from the links, he does have the right to refuse a change, so they can't discipline him for it.
However as you will also have seen, the bottom line is, if this is a change that could be justified as necessary for business reasons, they can force it through if necessary and as long as they do consult properly.
I think you both need to reflect realistically on what end result you can get. The employer has taken on a contract which requires working away. Therefore there is a business reason for the change they want to make. Your DP has a right to refuse but ultimately they could probably force it through, so he needs to think about how likely he thinks that is, and what he would do if that were to happen.
It's hard but it may be that walking away might be the best option. Think about what end result you both want and whether/how it's achievable.
If he is able to retract his resignation (or could realistically say he didn't mean it and it was a heat of the moment thing) goes through this grievance process, they still don't consult properly on this change or anything and/or discipline him for refusing (which is his right), he may well have a good case for constructive dismissal or unfair dismissal if they terminate his employment to force it through without proper consultation.