Ah, so it's YOUR wall, and they are plastering? Sorry, I had it the other way around.
Same thing applies though.... common courtesy.......... notwithstanding the legal position of interfering with someone's property.
I think you're wise to keep out of it for good, neighbourly reasons. One thing only comes to mind though. Highly unlikely, but, if any render comes off and injures someone (silly I know, but possible), then do make it clear that you are not liable just because it's your wall.
I'm not a lawyer, but perhaps a friendly word just to say you are aware of the work, and you're fine with it, but from now on, the render is their responsibility. Things like this keep lawyers happy for hours :o)