the first thing should be to put it in writing. she should write a letter stating what she is owed, why and when she should have received it and that she hasn't received the money.
the reason for this is that if he doesn't pay she may have a claim for unlawful deduction of wages, because she will at least be a worker. before she can bring this claim she must raise a grievance in writing within 3 months of when she should have received the money. a grievance can be quite basic and does not need to include legal language, but if she doesn't first put her complaint in writing the tribunal would reject her claim.
hopefully she won't have to bring a claim, but at least once she's written she will be able to, and once she has done so she would have until 6 months from when she should have been paid to make the claim.
also by putting it in writing it makes it clear she's serious about what she's owed and isn't just going to forget it.
(by the way, the above advice ignores the tax situation. but it may be that if she starts to raise the issue of tribunal proceedings the owner will pay up, rather risk having things scrutinised in a public forum)