Cat's from South Africa will need to undergo 6 months quarantine once they arrive in the UK. At the time my sister and brother in law moved over with Fuddy, the bill was roughly £700 for 6 months stay. (nearly 10 years ago)
You are permitted a weekly visit with your cat as well as the usual things included in the cost of boarding.
I do know that a) you can't transport your cat (put it on the air craft) until all relevant paperwork is in place, vaccination certificates, medical records etc including details of the cattery where the cat will be quarantined. b) if you have the details of the cattery, then you will also know how much it will cost and they will have worked out a payment plan already.
So, with respect to this lady, either you have not been told the whole story Julie, or this cattery is dodgy and should be investigated. If the woman hasn't told you the full story, then 1) she will have known the cost for 6 cats, as she will have had to pay for the transport of the same 6 cats and that's not cheap. Surely she would have done her homework as to what it would cost her to move countries?
2) the cattery will not have taken the cats in without her signing the relevant waivers etc (vets visits etc), and getting her to sign the payment agreement and/or taken a deposit (my sister had to pay upfront), 3) surely she would have chosen a cattery close to the city where she is now living, so will have visited... surely the money situation will have been discussed with her before that? And if she didn't I would question why.
If she can backup all discussions she had with the cattery and has relevant paperwork, etc, then it might be advisable to report the cattery to Defra.
http://www.defra.gov.uk/animalh/quarantine/pets/contacts.htm