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.

This is a tricky situation and unless there is absolute evidence that the maid has stolen it, I would treat the missing cash as gone forever. It also serves as a lesson to keep valuables locked and out of sight. If there is proof that the helper has stolen the money, then your friend has to decide whether she wants to sen the maid back or not.