If she decides on plastic I'd recommend she makes sure they are intended for use on blinds as other 'general' plastic rings might be cheaper but will not withstand the sunlight well and will quickly become brittle. These days they are usually clear polycarbonate, not white.
If metal is preferred then it's worth looking for reasonable quality ones rather than the cheapest to ensure they are smooth and will not have edges which will wear the cord over time.
Merrick & Day have a great reputation amongst curtain makers and will usually be able to send small orders in the regular post rather than the courier cost stated on the website which is for larger orders/bulky items.