The 'smells fine until pees' is classic build up so more strip washing needed! Has she gone back to basics and tried a load with full powder detergent then rinse, rinse, rinse until it has no bubbbles? Vinegar can help in bad cases but isn't great for elastic to keep doing strong soaks in it, I have also soaked a bad nappy in 1/3 vinegar for a couple of hours I think it was.
What is her washing routine (cycle, rinses etc)? What detergent and how much is she using? This is where we need to look carefully as it is potentially something simple that needs changing. Perhaps get her to jump on to answer all the questions! @LJmum
Bumboleo wrote:I'd be interested in more advice too! I tried vinegar on eye-wateringly stinky pre-loved bamboozles and it worked brilliantly but wondering what maintenance stuff you can do to stop having to buy vat loads of vinegar?

@Bumboleo I will ask you the same questions as above about your washing routine?
I tend to use vinegar at every wash, I put a splash in the fabric softener area with a few drops of tea tree oil. This way it is used in the final rinse and it is a fantastic natural urine neutraliser and the tea tree is a natural sanitiser. Some nappy makers are saying not to use it as there is potential degradation to elastic / pul however I've been doing it with nappies that are now nearly 10 years old (actually I think I have at least one that is older than that!) and I have had very few issues with elastic over that time but please be aware that that is a potential issue and make your own choices.
The other big mistakes that tend to happen are using too much detergent: 1/4 - 1/2 a dose depending on your water hardness is about right and using the wrong detergent, liquid is terrible IMHO for nappies as it leaves a residue and I think the 'non-bio' stuff that you have in the UK (this is only a UK thing the rest of us manage without!!) also can cause problems from feed back I've had on here. A lot of girls on here tend to switch to normal powder.
I hope that helps girls and please keep asking if you have more questions!