As per NWH.
My advice is to buy it new and to buy it from a reputable supplier such as Daqua.
The Chinese make cheap membranes with the promise of being an ultra low pressure membrane which they are not.
Buying second hand usually means buying an ro that needs new membranes and prefilters.
She will also need a di vessel and resin to polish the pure water off she has produced.
In winter she will need to protect the ro from freezing.
She will also need to store the pure she has produced, a used IBC tank is commonly used. She will need a transfer pump to pump the water she has produced into the tank in her sign written van.
She will also need a pole and most including me will recommend a Gardiner Slx with a Quickloq angle adaptor so she can swap between a couple of different brush heads.
Some hand towels and a couple of 'dirty' cloths (facecloth size) are also advisable to clean the hose as she winds it back onto her hose reel.
A uniform with her business name embroidered on the back is also a good idea to show customers she is professional.
It also a good idea to add some trad kit such as a good squeegee, applicator, a scraper and a bucket.
How is she going to gain customers?