Not really sure which documents are involved ,my solicitor is looking into them, the "Wypis Z Rejestru Gruntów" is in the sellers name.
If she's entered into the Rejestr Gruntow that doesn't mean she owns the house on the land. We had the same issue in my family where the land was registered to my grandfather (from before WW2 - he died in the 1970s) but the house was owned by by grandmother (who died in 2001) so my Mum had a bit of paperwork to do in order to tidy everything up. In our case it was pretty simple, but it could clearly get much more complicated. Personally, if the price was right I'd sign a pre-agreement and make it a condition of completion that your vendor completes the KW process.
Also, I also bought a cooperative property in a block at a very low price and haven't bothered getting a KW for it - it would require a visit to a notary and a wait of about 3-6 months for them to write it in. As part of the sale I got written in to the register of members of the cooperative which is proof of ownership.