what mattress to choose for my grandmother

my grandmother is 91 years of age and slim build , weighing 9 stone, i would like to buy her a new mattress , im afraid to say, the mattress she sleeps on is probably 50yrs old!! So it is very moulded to her shape, she is used to a very soft mattress, and i would like advice as to what to replace it with, especially as she is very anxious that a new mattress would be too firm and give her a restless nights sleep.Many thanks.