She doesn't want to go there again, she's adamant about that. It hurts her every time she sees her mum, because her mother looks so ill it just reminds her of how she's ruining her life. She does love her mum, but she doesn't respect her, which is fair enough. She is old enough to choose not to get into a car with her mum, and I think in the future she won't, it's just last Sunday she obviously didn't know what her mum had planned until they were actually moving and driving towards mum's town.
We have always insisted that mum came to our town to see her, and she did (albeit very very sporadically), and she would come on the bus so there was little chance of her being able to take SD away. There are buses on a Sunday, but they aren't as frequent, so we're guessing that she asked her friend to drive her because of that. On previous visits, and because my husband told her she is not to drink in front of SD (she had on a previous visit and SD got really angry about it), she has only stayed for an hour and has been able to catch the next bus home (which are half hourly weekdays and Saturdays), but on Sundays they are 2 hourly and that's probably too long for mum to go without a drink.
I think that the next visit will need to be supervised if she comes over in her friends car again.