Since I know Boxtops is well-versed in this area and had covered off the key points, I've just sat back and watched, thus far. However.....
There is no surity to getting one's way over this. The best bet is to follow the process of raising a grievance over the employer's proposed action. The employer should then follow an appropriate process that should end up "justifying" their proposed change against at least one of the eight criteria contained in the link in boxtops first reply. Read down through the article to find them. Your partner should bring to the table for the grievance discussion all the flexible things he has done, like being willing to work the weekend shifts when others won't do them.
The outcome should either result in the employer agreeing with the grievance, OR REJECTING IT ON ONE OF THE EIGHT VALID REASONS. IF the employer fails to operate the correct process, your partner may then have a case to take the issue back to ACAS, and the employer might then be persuaded to back off, because of the implications of a failure to deal with the grievance properly.
The partner should take notes of face-to-face discussions he has.
The "nuclear" option discussed above, resigning and claiming constructive dismissal, is not to be preferred, as you don't get your job back, just money.
That's my best offer, I'm afraid, and as I said at the start, it might not work, but using a grievance process should not result in long term damage.