It seems to me the house was built illegally. (Apologies if not so). The reason I say this is I know of houses in Alicante region that have been built illegally on rustic land (the rustic land is not the problem). The law changed (several times) and oddly enough even illegal houses can now be added on to an escritura to say they 'exsist', but that does not mean they are accepted, (only to pay local taxes) and although if you have a cash buyer who either chooses not to use a soliciotr or not a very good one, this could be overlooked, if you have mains elec & water - from the escritura appearing to be a legal property, but if the buyer knows what they are looking for / there solicitor does his job properly they will be advised not to buy. If someone wants a mortgage, it will not be possible.
I have a friend who bought a house - the last one out of a village, on rustic land, though it's part of the village & has all the services. The house is about 50 - 80 yrs old. When she bought it was changed to urban under the townhall changes of boundaries - and when she connected to mains water & upgraded the elec, and installed a 'proper' bathroom, it was given a cedula of habitation.
I know more who bought illegal houses, that technically didn't exsist. So they officially only bought a piece of rustic land with connection to scanky water from an irrigation resevoir ( with normal looking water meter)! (They got mains electric before the law changed - though not to a "house" ). After 4 yrs of waiting, they had it added on to the escritura, which means any unsuspecting buyer who chooses not to use a solicitor, could asume it legal & buy. Adding to the escritura is better than not, for the owner / seller but it doesn't make it any more legal / easier to sell to someone who has a solicitor / needs a mortgage.
The rustic land is not the problem - We have built a few houses here in Alicante & Murcia regions - all on rustic land, which has stayed rustic land, but with building licences, architect, connection to atleast mains electric & water ( only water deposit possible on some) etc etc, there is no problem to gain the habitation certificate.
25yrs is not a very old house, and if built new should have had building licence etc - does your friend know anything about this?
Maybe it was an old house/ruin that was demolished & rebuilt? - but if this was done under a 'project' /proyecto again there should be no problem. If no project this could cause a porblem. Even some ruins cannot be rebuilt without certain permissions etc.
I suggest your friend tries to find out a bit more of the house's history. It seems not possible that 90% of all Catalunya houses are in this situation, but maybe 90% of the area her house is in, if any of the above mentioned applies! - In a rustic area - when a few people start building illegally / renovating ruins etc etc, others soon catch on to the idea.