My friend's mum discovered some money missing, and saw some notes hidden in between her helper's clothing one day. how can she speak to the helper about it? there are only 2 other adults in the house.

I think that she should not speak to the helper about it until she is sure that it is her money. They should probably monitor the situation for some time and keep track of the amount of money they have in their cupboards (or wherever they keep their valuables) and see if it happens again. To be extra sure they can take a photo of the cash they have that clearly states the serial number of the notes and tally it with the money that the maid has. When it happens then I guess it's time to send the helper back? Confronting her without evidence now may affect the working relationship. However if is is to be done, she can try to indirectly give her a warning to deter the incident from happening again. For example your friend's mum can just casually mention to the helper that she has lost some money and ask if the helper saw it. This will alert the helper and let her know that your friend's mum does keep track of her own property. Also gives the helper a second chance if she really did it.