She needs to express just enough to keep her feeling comfortable - any more and she will stimulate more milk production which is not what she wants. She can hand express into the shower or use a pump (and maybe mix it with her DD's formula?)
Also - her midwife or HV should have told her this - she needs to watch out for any lumps or blockages which can lead to mastitis. If she feels any lumps or blockages she should massage the lump towards the nipple and contact her midwife/HV for advice.
I don't know: its a good idea I suppose isn't it, rather than throwing it away; but then she would need to express enough to make it worthwile which might make for a lot of extra work.
I really would just like to say something helpful, whilst respecting totally her decision not to breastfeed. She breastfed her DD1 and found weaning to go back to work a nightmare. She knows I breastfed all three of my dcs long term; I don't want her to think I am trying to convert her...