My flatmate is using washing powder for handwashing in our washing machine. For various reasons (the usual tedious flatsharing stuff) I tend to like to stay out of her business, but if this is likely to damage the washing machine I do need to say something. Is it?
You mean clothes washing machine, right? It won’t damage it but who knows what it will do to the clothes. A washing machine just swishes it around and then pumps it out the drain. Nothing water soluble (except something like acid) should damage it.
Yes, clothes. To be honest, I don’t really care what it does to her clothes - I’m not using it for mine, though. 
Was worried that it would maybe produce too much foam and cause a blockage.
Soap for washing dishes by hand will quickly overflow a dishwasher and will expand to cover most of the kitchen floor.
At least, that’s what my ex-wife told me she was cleaning up when I got home that night.
QFT. My brother did this once, and one of my college room mates would have done if I wasn’t there to stop him.
Likewise, Mr Bubble in a jacuzzi is not a great idea.
You must have to use a lot to get that kind of foaming. I’ve washed loads of clothes using dishwashing liquid in a pinch. Only use about two tablespoons of dish soap for a load, though … you definitely have to scale back the amount.
I have always assumed that she filled the little chamber. You’d have had more luck getting a Sprigfieldian to discuss Armand Tamzarian than you would getting my ex-wife to discuss the Dishwasher Incident.
I don’t even like powder that’s specifically made for washing clothes. If you put in a little too much, some of it doesn’t get dissolved, and it gets caked into little out-of-the-way parts of your clothing. Stick with liquid.
I think it was the dishwasher, and not the clothes washer. My roommate did this with the dishwasher as well. The hand soap stuff is really concentrated, so when you fill up the little container, it quickly suds up until it covers a fairly decent sized kitchen floor. That reminds me; I have to find the pictures we took of him cleaning up bubbles using the pots and pans.
Foam won’t cause a blockage. It’s too…foamy. It might possibly not all rinse out if it’s too much but it can’t cause a clog.