Was the marriage something they had discussed or was it something she assumed/wished?
She has two problems:
This is the first one. She is living with a man who is using her and who does not live her or respect her. She possibly neither loves nor respects herself. She invests far too much in relationships with men is putting it mildly.
'He is v careful that she doesnt pay for anything structural so that she cant claim that she has contributed to the house's value if they split.' Says it all.
He has his long term plan all laid out. She has hers. Never the twain shall meet. She will come home one day to find her replacement waiting for her.
Her second problem:
'holidays, presents, house decor, car costs, eating out, takeaways, dvds, electrical eqipment for the house. she also spent a lot on white goods for her old flat which she left behind when they moved into his Mums.' She has a habit of living beyond her means, and that is a problem that is not her BFs to fix by paying her debts. There is no way the BF has any duty to pay off her debt. This habit will ruin her life if she doesn't fix it herself ASAP. She should not spend one more penny on house decor for a house that will never be hers. She should not spend one penny on electrical equipment (do you mean TVs or music stuff, or electrical wiring here, or kitchen appliances?) Anything she has bought that is not nailed down she should try to sell. It's hers if she bought it. Eating out and takeaways will also (1) make you overweight if you do it too much, (more than once a month), and (2) burn a hole in your money.
Her basic expenses are all she should pay -- transport and some food. No more clothes or shoes or handbags. She should be looking for a better paying job.
She does not seem to really value money or take care of the things that it buys. This may sound terribly materialistic, but here's my take on it: If you realise that the money you have to spend is the result of the work you have to do and the effort you put in to keep that work, then you will value that money and respect the things that that money will get for you. If you have little or no respect for your competence at your job and the money it brings, (i.e. for your own input into your own life) and you value things outside of yourself, like 'A Man', that's the only thing that you will consistently pursue, often to the detriment of your finances. She was willing to move 30 miles from her job for this man.
This man seems to understand money. She needs to sit him down and tell him what her spending plans are -- she will spend what is necessary on transport for her job. She will not buy more than 50% of groceries. She will not spend another penny on anything else. In return for the housework she does, the BF will assume a portion of the house-related charges that are now split 50-50, so he should pay 75% instead. This is what a maid and cook and laundry service (and an occasional call girl) would cost him, and it is reasonable.
Receipts will be necessary to calculate who owes who what at the end of each week. She will make a list when she goes shopping and she will stick to it, and she will not eat out until there's a huge dent in the debt. (25% reduction) She will learn to cook. She will check her bank balance every day online, and count every penny in her handbag. She will become aware of the exact amount of her debt and be able to tell what it is on any given day.
'I think that after 8 years together it would not be unreasonable for him to pay off her debts (at 20% interest) with his savings and trust her to pay him back. Their joint finances would then be improved in the long run.' There are no joint finances here. The BF has long term plans that do not include this unfortunate self-delusional friend of yours. He has no duty to pay off the debts of someone he has led on and dictated to, unfortunately.
She needs to take the wool off her eyes and start thinking in terms of Me and My Money. The man is a complete twat and there is no future, financial or familywise, with him, for her. If she had one ounce of self respect she would be able to see that. Please don't encourage her to find any solution to the mess she has got herself into that involves sustaining the illusion that there is any hope for this relationship of hers.