You need to be a little more specific if you expect any helpful answers here.
What do you mean by "property tax"? To whom have they been paying it? Presumably it's been getting paid by direct debit from their bank account, so perhaps the first point of help may be their bank, who should be able to explain to them what "tax" it is. If it's local rates, known as IBI, then they need to go to the town hall with proof that they sold the property (deed of sale) to get this tax switched to the new owners. And then cancel any direct debits!
If it's non-resident tax paid to the tax department (Hacienda) then I suggest they contact whoever handled the sale for them, or their legal representative in Spain, because someone must be submitting a 210 tax declaration on their behalf for that to happen.