Best mattres for my wifes back

My wife's height is 5ft 4inch and she is 13st years ago after having our daughter she has had a really bad back in her lower spine, she mostly has to sleep on her side as if she lies on her back she ends up near to tears trying to get up in morning and sliding out of bed as she cant starighten her back up straight away most mornings, matress we have is an old memory foam and i know its no use, im 6ft and just over 16st, but im not bothered about myself just would like some help in getting something that could help her with her sleeping, i dont expect it to stop her pain but i really have no idea what to look for, thanks for any reply in advance