Softened after just 8 months

My Aunt purchased a twin mattress from Mattress Firm and in 8 months the depth is 1 1/2 inches deep and she only weigh 115 lbs ! She wakes up with an awful backache every day ! We have contacted Mattress Firm TWICE already and they are a very bad company to do business with ! After many letters we finally got someone to say we needed a new mattress, however they are only allowing her a $300. 00 credit ON ONLY ONE MATTRESS !!! What about their motto "Happy or your money back". We are not looking for money back [if so, we would go to a different store !] but all we want is a better mattress even if it's only one !!! My Aunt is in financial difficulty at the present time since she just had to admit my Uncle to an Alzheimer's facility and there's not much money coming in so I had to move in to help her out.. She unfortunately chose a "cheap" mattress [Hampton Rhodes ---which cannot be contacted!] so I was wondering if you could give us some guidance in dealing with this matter. By the way, if you read the reviews on Mattress Firm, you would not want to do business with them ! Wish she had asked me to check it out before the purchase [she doesn't have a computer] . Where do we go from here? Please give me some insight, Thank-you, Karen Stapleton cell # 407 488-7870 or my email karen.stapleton@att.net