I kind of think saying nothing is a bit passive-aggressive. My sis-in-law used to let her son (at about 2 yrs) do just about anything in other peoples' homes. Until he visited me. When he tried to break something, I'd tell him off and take what ever it was off him. When he stood on a book, I'd move him and put the books away. When he squeezed his juice-box on to the carpet I threw it in the bin. I just had had enough, I wasn't her nanny, and it isn't her house. She now knows that he's misbehaving at other peoples' homes, whether he behaves that way at his own home or not, so she stops it. If it's unacceptable to you, do something about it. Personally, I'd put aside some crappy old toys to bring out just when he's there and let the mum know that due to him being destructive they're just for when he's there. Tell her you really like her company, and her child's, but that no one can afford that kind of behaviour. And I do second the suggestion that you play outside. Oh, and if she asked 'is it ok that he does (whatever destructive thing he's doing)?' say 'No, it's bl**dy not!'!!!
A Long Journey is a good journey