Twiglet - tough one. How much do you like the mother?
We had similar situation. DD loved playing with this little boy but he could be really naughty and right pain in the .... Mum and I got on like house on fire. My biggest worry was DD copying his behaviour - which of course she did at times. Mother knew her DS had certain "problems" which she did try to sort out. I resolved to explain to DD that just because xxx did whatever she couldn't and that she knew that xxx was silly sometimes - silly being the least offensive word I could come up with.
When in my house or my care I would treat him like her - no messing around on my turf (makes me sound like drill sergeant . It seemed to work. The family now live abroad but has just visited. The boy has plenty of respect for me and behaves well in my presence (what you do round your house is none of my biz). He has also changed full stop - obviously nothing to do with me.