If I were your dh I'd be making sure I'd got as many copies as possible of any email communications, letters, phone messages, intranet pages, web pages etc of anything to do with the redundancy process, so that if anything did happen to him, he's got as much ammunition as possible.
I'd also try to get copies of things like the job description and linked in pages from the three people that have been made redundant, especially the one that you suspect might be after your dh's job, just so you can see what their skills are and how they match up to what your dh can offer.
Another avenue to look at is your insurance - do you have legal insurance included with it? Many do... They will often be able to help you - even if they can't represent you all the way, they can give you advice on the basics of what the process should be, what to look out for and so on.
I'd also ask advice on at what point you want to start to drop terms like unfair dismissal and not following the proper process into the conversation. Do it too early and they have a chance to go back and correct themselves so you might not get compensated if they were doing things wrong, do it too late and you might not make yourself safe if it is the case. I don't usually reckon on trying to make money out of situations but if you're potentially being made redundant having been told that you were safe, which must feel like a huge kick in the teeth, then I think it's fair enough.
Can your dh formally express an interest in his own job (and any others?) Particularly if he has a good solicitor, then they might be able to construct the letter to point out the farcical nature of the issue - that if his job is available for the other person then it's not a case of redundancy as the job is available - which would make the HR team think twice about giving it to a different person (even if they have to interview your dh and the other person for the single role, and enable your dh to be the best person for the job).
It does sound a crazy scary situation to be in - so keeping fingers crossed that they come to their senses and let your dh keep his job.