Rubber rings.

If all else fails, buy an inner tube for a wheelbarrow wheel ( 8 x 4 ) and inflate to about 5 pounds / square inch. Doesn't need a lot of pressure in it.
Put it in the chair with the valve facing down!

It may not look elegant, but will probably do the job, and last better than a more expensive "medical" product.
If it looks too utilitarian, make / knit a cover for it.

Have you tried The Disabled Living Foundation? They advise on lots of items and can be accessed online so friend can maintain independence.

I don't know the answers to the questions on care or who supplied the chair. I suspect she will have accepted minimal professional help as she's independent to the n'th degree.
But thanks to the addresses of places that supply them.
I doubt if I'll be able to influence her in any way when it comes to accepting advice. I can but try!

My mother sits in a wheelchair for several hours daily and alternates with a dual motor armchair. She was assessed and a gel cushion was supplied. She was advised that rings are no longer recommended. She also has a pressure relieving hospital bed.

Could a neck cusion help i wonder ?.

I have found the link to the Waterlow Risk assessment scorecard which may be helful when talking to whoever your friend contacts. the only term she might find confusing is "terminal cachexia" which relates to end of life care and is therefore not relevant to her gelovationseurope.com/pressure-ulcer-assessment-the-waterlow-scale/

The HV should either be doing a proper pressure damage screening assessment or referring on to someone who can. I am shocked, but not surprised by the poor advice given by her HV...Is she getting any help in managing her continence? Is she under any specialist team? Who got her the wheelchair?

I have just bought my mum a replacement rubber ring from Amazon.

If you google lots of different types come up like this one

I agree with riverwalk though. There are lots of different types of cushion or mats now that will relieve pressure and not cause soreness. Some even have personal technology wrt urination too. I would suggest the continence nursing team might be able to advise too, they can be accessed through the GP service.

Dunlopillo make foam rings, for which covers are available. Also inflatable ones with pump on Ebay.
Dunlopillo ring

Inflatable ring

Not hard to find, eg www.amazon.co.uk/ring-cushion/s?k=ring+cushion?&tag=gransnetforum-21

Google Valley Cushion.
Used after childbirth - may be appropriate?

I'm sorry for your friend - sitting on a gradually perishing rubber ring for years can't have done her much good.

She needs a proper pressure-relieving cushion for a wheelchair and should speak to the Occupational Therapist/Community Nurse.

Also, no matter what she's sitting on she shouldn't sit for long periods of time without changing from chair/to wheelchair/ to bed.