custys dont like change, i dont understand how a soft brush can scratch a windows, if that was the case you wouldnt wash ur car at a car wash as it would end up covered in scratches lol a lot like you to basically earn ur money and that means working up and down a ladder, i have just bought a wfp system and am using it now, and have not had one custy complain about that over trad,
you might have to say to the women that you will give her a demo on wfp by you and do the best clean ever
if she is still moaning you might have to just do it trad, or drop her
the way i look at it is if i fall and break my leg custy aint ging to pay me to not clean ther windows and put food on my table, wfp is safer