A firmer mattress is likely to help - given her pain also comes from her neck also I would suggest checking she has a good pillow as well. I don't know any good brands specifically.
While taking courses on back and neck pain I can clearly remember one tutor saying "I'm amazed at how many people will pay thousands for a new bed but not a couple hundred for a course of physio". No idea what treatment your wife has had - but certainly a strong course of core stability work for her back and neck muscles are well indicated in hypermobility, and can be cheaper than a new bed. consider doing both however as a saggy bed is no good if both of you are getting pains.
all the best