Error with Property Deeds...

Could you tell me if it is in any way possible to alter the names listed on the deeds of a Spanish property?

The question arises because my sister is trying to return to the UK from Spain, where she has been living with her partner (they are not married) since 2004, whilst he wishes to stay in Spain. She funded the original purchase of the land on which they subsequently had a house built, and also funded the building project - and has a complete document trail to demonstrate this. However, on working through the documentation with a view to returning to the UK (alone) she now finds that whilst she and her partner have a joint mortgage, for whatever reason, her name was left off the deeds.

She was shocked when she found this out - only recently - and realises that it was a huge and terrible 'administrative' mistake, but she (naively, it seems) trusted those involved in the purchase process and just doesn't know why her name was included on all other doucments, but left off what she now finds to be the key one. Only her partner's name is shown.

She invested all of the money from her UK home and business in the Spanish property, and will have lost her life's savings if the situation can't be resolved.

Given that she has a full paper trail to show that it was 100% her money which funded the purchase of land and the build, she would like to challenge what she sees as an administrative error. What are her chances of success?

Any advice would be invaluable.