I think that a lot of people have to resort to it, I can think of circumstances where I would have done it.
However this child has had no choice, she hasn't tried it to know the choice and her mother is making her scared of other children by assuming that she will be bullied- with no evidence that this would happen. If she is going to do it she will need to let her child socialise with home schooled groups, Brownies, etc and she is just as likely to meet bullies there- human nature is the same the world over and they are not all 'little Angels' because they don't attend school!
It also sounds, from tne term 'unschooled' and her views on 'the system' that she will have the autonomous approach where the child chooses what to do ( or generally what not to do) and then somehow get herself to Oxbridge!!
This is the type of home educator who won't have a written plan and is less likely to allow home visits. They also slip right through the net if they never get into the school system.
I wouldn't worry about it too much. There is a lot to be said for not starting school until 6/7yrs. She is making a huge commitment if she intends to do it for 11yrs. Her circumstances may well change, she may get bored, find it too much- and since she is intending to do it in benefits I can't see how she will be allowed to- she will be expected to work.