I too used to work from home and, like TillyWhiz, found that people didn't respect that I was doing 'real work'. The biggest offenders were my elderly parents who would drop in at any time. Sometimes I would be on a conference call and
my Mum would 'join in'! I was a single parent bringing up three teenage boys and this was my sole income.
Eventually, I had to have a gentle but firm word with my parents and my Mum, bless her, did take offence for a few weeks. After she had time to think about it, she did eventually understand that I had deadlines to meet and a living to earn.
Luckygirl, maybe you could suggest to your daughter that she type out a schedule of availability and give her friend a copy? That's what I did and no-one took offence as it was written in a positive way. Your daughter's friend would have to be very brazen to ask her to babysit once she has a copy of her work schedule. It may help.