Trying to avoid benzo addiction by using various benzo's, is like trying to avoid opiate addiction, by using various opiates...it ain't gonna work.
But Sparky knows you know that...now.
She'd advise (as the OP is in the UK) that they go see their doc, they're the expert, they can give them a sensible reduction strategy. Making sure they're safe.
To be honest, asking a benzo addict to reduce themselves is like asking a heroin addict to reduce themselves, they're addicted, when the going gets tough, they'll probably use more than they should. (another damned analogy

)
With a structured taper from the doctor they will find it easier.
Maybe try to get some help for the anxiety they suffer, isolating will just make the need for self medicating more real, after all, that was the problem that seemed to start this, so get support for it.
Zopiclone, whilst not as addictive (apparently) as benzo's is now being found to be more abusable than first thought, but it's still an option when he gets his dosage low. Although they are only indicated for short term use. It's usually used to treat insomnia. It can give a nasty metallic taste in the mouth, but this goes, eventually.
Look at a sleep routine, do the same things every night at the same time. get plenty of exercise, a tired body will sleep. But it's essential to get up at the same time every morning, regardless of only having had 2 hours sleep. You don't wanna turn you body clock round, and end up sleeping through the day. try reading, listening to audio tapes. If you can't sleep, don't lay there getting stressed, get up for an hour, then try again, eventually, you will sleep.
Again, the doc can advise better than anyone here, they know SWIY.
But please, give yourself the best chance, get help for the anxiety. Yeah, Sparky is saying it twice, if you don't, you're gonna be fucked.
Treat the problem, not the symptom...and stay safe. Benzo W/D can be dangerous, so be very careful.
Sparky.