Not to berate OP or his mother, but this is a public board and odd transit documentation issues arise all the time. There are always two issues to consider and these are as much part of trip planning as hotels or anything else. It is 100% the sole responsibility of the passenger to determine that s(he) is properly documented. TA's and air carriers have no responsibility to warn in advance.
1. What will the air carrier do? Carriers check documents solely because they are subject to hefty fines and the duty to return the passenger to her origin if not admitted for lack of documents.
2. Often, local immigration authorities may be lenient. But, it does not matter if one does not get to the destination.
3. IATA populates the TIMATIC database for its member carriers for just this purpose. What you see through the various public interfaces linked above is what the check-in agent will see when you show your documents. IATA gathers the information from foreign ministries (or equivalent) and then rewrites into short paragraphs and asks the foreign ministry to recheck. If TIMATIC shows a visa as required, the likelihood of a carrier boarding a passenger unless it makes a mistake is close to nil. Some countries such as Germany do maintain a hotline which carriers --- not passengers --- may access, but that is a time-consuming process and is not direct from a counter agent somewhere in the world.
While this is all an annoyance and can be a costly and logistical mess, the when and why of visa and other documents is not always clear and if there is a remote doubt in your mind, ask yourself whether it is better to obtain the visa (presuming that there is not some issue with that) rather than leaving an elderly relative stuck somewhere, in this case Australia.