I suggest you get your friend to trawl through this little lot.
http://www.businesslink.gov.uk/bdotg/action/de tail?r.l1=1073858787&r.l3=1074019927&r.lc=en&r .t=RESOURCES&type=RESOURCES&itemId=1073792400& r.i=1073792401&r.l2=1073876974&r.s=sc
I can't explain it all in under 2000 words limit on here - nor do we know the exact circumstances. The straight answer to your question is that 1 person can be impacted by redundancy - though employers have to be really careful about where they draw around the 'pool' (read the gumf to see what I mean by the pool).
If he is keen on saving his job, then offering to be redeployed to the old role (and salary) might be a good tactic. The employer must follow a fair process (outlined in the gumph) including consulting with the employees impacted - what you describe does not sound like proper consultation to me (if they have discussed how to carve up his job with others).
Also worth ringing ACAS for their specific advice on his case.