The Oryx Hotel at the Hamad International Airport is indeed located inside the main terminal before passport control & customs. The hotel can be booked online and the rooms are available for a maximum of 24 hours. The hotel is pricey (random dates had a rate of 950 QAR) and can be booked online. Your mom would not need to pass through immigration or customs but would need pass through the transfer security area of the airport. I am not sure if QR (assuming that is the carrier she will be on) will check a bag all the way through with a 26 hour stopover - you may need to check with QR on that. Note that, if they do check it through to the final destination, she would not have access to it during the layover. Also, note that when reserving a room at the transit hotel, the person checking in needs to present the physical credit card used when booking the reservation.
If you go to
Home - Oryx Airport Hotel you will see more information. They also have a link where you can email specific questions to the hotel. I did that recently and they were swift in their responses.