Ladders are legal only if the risk is assessed to be minimal or no other alternative is available (not having a pole systems does not count in the HSE regulations).
The HSE are actually very specific when it comes to window cleaners. The tricky part is that assessing of the risk. The trouble is whenever you climb a ladder and go over a certain height (3m) there is real risk of serious injury. Because most window cleaners have got used to this risk they can tend to ignore it, but it is still there and you have to prove that you have put in place measures to contain this risk (tying the ladder, using an approved fixing device, having someone foot the ladders) or remove the risk by using am alternative method (WFP, MEWP, Scaffold).
If you go onto the HSE website and look up the working at height regulations you will find all their advice. Something that your potential client needs to think about is also the new corporate liability laws that would hold her responsible for any injury or death that occurred on a premises that she has responsibility for.