Discovered a week after the event by another friend of DD's who was staying over. The name of a pop star they were listening to scribbled in pen on the bottom corner of the door, side facing the wall when door open.
DD's sleep over friend noticed when she was lying on the blow up mattress. DD said it must have been friend from last week and sleep over friend confirmed the handwriting. DD had left her friend colouring while she went to the toilet.
The door is stripped wood so we can't get the pen out (any tips for this welcome).
I don't want to ban DD's friend from the house and I get on with the mum. Do I say anything? Children are aged 8.