Hello. You can go ahead with a good lumbar belt that covers the entire lower back for support. It can be the simplest one. Brand does not matter.
Also, the best thing to be done at this time is ask your mother to hold in her belly button in as 10-15% contraction and maintain that during her day to day activities. That is while standing, getting up from bed, sitting on a chair, standing in the kitchen and especially while doing household chores and liftiing objects.
The reason to do is- her core muscles are weak and that is the real cause of back pain, By doing this contraction, she would minimize the risk of worsening her back pain and GIVES A NATURAL SUPPORT TO HER CORE WITHOUT WEARING THE BELT FOR ALL DAY LONG. Feel free for providing further details and reports. Thank you.