house rabbit chewing carpet - help needed

A friend in London has 2 rabbits. He has had his female bun for a few years. Recently he has adopted a neutered male for a companion for her, and with a little support he has managed to bond them. However, this new boy is destroying the carpet. He has tried chew-stop (on advice from his vet) but bunny still chews.
He has lots of toys, cardboard boxes, a digging box.....
I suggested hutching bunnies when not supervised, but the male goes crazy to get out.
It is a good home and the guy really loves the buns. Money is no problem if he can sort this one problem.
But landlord is insisting that he buys a new carpet, and is not happy.
Has anyone got ideas please??
Both rabbits are neutered. female about 5-6, male about 3-4.
If this problem cant be resolved, owner may have to rehome.... but this will be a shame.

Somebuns just can't be stopped. Biscuit is a determined digger and will destroy carpet, we've tried everything and he destroyed the carpet in our old living room and we had to lose part of our deposit in our rented house to replace it when we left.

I would say bun is just being a rabbit and its digging habits from its wild genetics are showing strongly

How persistent has he been with the chew-stop?

Ours used to do this and we used some similar spray along with clapping/chasing every time they chewed. It probably took 4-6 weeks of perseverance, but they don't do it now.

I think he used the chew stop on just 2 occasions. The rabbit only seems to chew the carpet in the early hours of the morning, and never when the owner is present. He came from a rescue (dont know which one) and had been a stray and then living in an outside hutch before becoming a housebun. He has lots of toys, boxes, etc. Owner does not want to pen or hutch the rabbits at night or when he cant watch them.
I will advise he persists with the chew stop. Many thanks

We've found vinegar works better than chew stop. You have to be persistent. Every time the bunny does it make a particular noise (and only use it when bunny is being naughty) and sprinkle vinegar on the area of carpet he's digging. It took us about 10 weeks of this to stop Buckley trying to chew the puppy panels (they were penning the TV in, not him! He has free range!) and eventually he stopped doing it.