Legally he doesn't have to make that choice.

The employer can suggest he reduce his hours either permanently or temporarily but cant insist upon it. (and clearly doing it over such a short space of time whilst "understandable" in a crisis is neither the correct way about it or helpful). He "might" suggest a temporary reduction in his working week of say 1 or 2 months with a contractual right to (a) return to his FT hours at end of this and (b) any redundancy to be based on FT hours. (b) has some risks attached.
 
If the individual doesn't want to do that then the ball is very firmly in the employers court. They either make the individual redundant (and it is certainly not voluntary) or make savings somewhere else.

Not accepting these changes doesn't affect his ability to be made redundant at all. Theoretically if employer was offering enhanced redundancy terms (they don't appear to be) then they could offer more for voluntary redundancy.

Any unreasonable delay in paying out redundancy payments can be challenged in the courts but f course the employer has to have the cash to be able to pay...

In reply to Keith:

That's really helpful, thank you so much for your time and advice on this. I've passed that on to him.

In reply to Keith:

Agree with Keith.
Given the apparently precarious financial state of the Company your friend will be in a difficult situation whether he stays or whether he is made redundant - getting paid or actually receiving redundancy pay will probably be a real problem.
He should leave the ball firmly in the employer's court and wait for them to make a decision/proposal. In the absence of proper process/consultation he could well end up in a situation of "unfair redundancy" qualifying at the same time for "unfair dismissal" and "redundancy" - but this is only "good news" if he can get the money.....
Time to look elsewhere in any case?

In reply to Ray:

Worth adding that there's a partial safety net even if the employer ends up insolvent, but only for recovering  basic / capped / restricted sums for statutory redundancy and notice -  not unfair dismissal awards etc - and only after a lot of rigmarole and form filling and waiting. But useful to have at least something there, potentially.