Jister is spot on here....what do you care if the plastering is rough on her side, she has to look at it not you. By using corner beads you will have a perfect finish on your own side with a clean edge. As the blockwork is built up get the blocklayers to lay on a good thick coat of render after every four corses that has added waterproofer in the mix, by using drop boards the builders can catch any mess that falls onto her side. The wall will be watertight and also ready to receive a finished coat of plaster when she finally gets sick and tired of looking at an unsightly scratch coated wall. This solution is easy to do and should eventually bring her round though personally when she decides she wants her side rendering I would tell her to **** off ....my extension is perfectly dry and warm, you had your chance.